Package Manager is hollow. I did a lot of research on the internet and couldn’t find a solution
Unity 2020 and 2021 versions available, both clean fresh installs. Both have this problem.
Check the Editor and Package Manager logs - here is how to find both:
They may provide more info as to why the Package Manager won’t work for you.
Also, make sure that you don’t use any VPN or Proxy.
.
I don’t understand much?
vpn and proxy I dont use
[2021-05-30T20:40:41.986Z][INFO] Command-line: 'C:\\Program Files\\Unity\\Hub\\Editor\\2020.3.10f1\\Editor\\Data\\Resources\\PackageManager\\Server\\UnityPackageManager.exe' -s 12552 -p 49202 -l 2
[2021-05-30T20:40:42.004Z][INFO] Creating server instance
[2021-05-30T20:40:42.110Z][INFO] Starting server
[2021-05-30T20:40:42.129Z][INFO] Server started on port [49202]
[2021-05-30T20:40:42.210Z][INFO] GET '/health' <-- Request received
[2021-05-30T20:40:42.212Z][INFO] GET '/health' --> 200 (2 ms)
[2021-05-30T20:40:47.348Z][INFO] GET '/project/registries' <-- Request received
[2021-05-30T20:40:47.386Z][INFO] GET '/project/registries' --> 200 (37 ms)
[2021-05-30T20:40:50.667Z][INFO] GET '/project/registries' <-- Request received
[2021-05-30T20:40:50.682Z][INFO] GET '/project/registries' --> 200 (15 ms)
[2021-05-30T20:43:46.754Z][INFO] GET '/packages' <-- Request received
[2021-05-30T20:43:46.760Z][INFO] GET '/packages' <-- Request received
[2021-05-30T20:43:46.763Z][INFO] GET '/project/dependencies' <-- Request received
[2021-05-30T20:43:46.766Z][INFO] GET '/project/dependencies' <-- Request received
[2021-05-30T20:43:47.041Z][INFO] Resolving dependencies using SAT solver
[2021-05-30T20:43:47.049Z][INFO] Resolving dependencies using SAT solver
[2021-05-30T20:43:47.459Z][INFO] GET '/project/dependencies' --> 200 (691 ms)
[2021-05-30T20:43:52.327Z][INFO] GET '/packages' --> 200 (5566 ms)
[2021-05-30T20:43:56.757Z][INFO] GET '/packages/com.unity.addressables@1.16.19' <-- Request received
[2021-05-30T20:43:56.765Z][INFO] GET '/packages/com.unity.animation.rigging@1.0.3' <-- Request received
[2021-05-30T20:43:56.773Z][INFO] GET '/packages/com.unity.burst@1.4.8' <-- Request received
[2021-05-30T20:43:56.775Z][INFO] GET '/packages/com.unity.cinemachine@2.6.5' <-- Request received
[2021-05-30T20:43:56.794Z][INFO] GET '/packages/com.unity.formats.alembic@1.0.7' <-- Request received
[2021-05-30T20:43:56.807Z][INFO] GET '/packages/com.unity.mobile.notifications@1.3.2' <-- Request received
[2021-05-30T20:43:56.808Z][INFO] GET '/packages/com.unity.polybrush@1.0.2' <-- Request received
[2021-05-30T20:43:56.809Z][INFO] GET '/packages/com.unity.probuilder@4.5.0' <-- Request received
[2021-05-30T20:43:57.562Z][INFO] GET '/packages/com.unity.addressables@1.16.19' --> 200 (805 ms)
[2021-05-30T20:43:57.566Z][INFO] GET '/packages/com.unity.purchasing@3.1.0' <-- Request received
[2021-05-30T20:43:57.609Z][INFO] GET '/packages/com.unity.mobile.notifications@1.3.2' --> 200 (802 ms)
[2021-05-30T20:43:57.612Z][INFO] GET '/packages/com.unity.purchasing.udp@2.0.0' <-- Request received
[2021-05-30T20:43:57.731Z][INFO] GET '/packages/com.unity.animation.rigging@1.0.3' --> 200 (966 ms)
[2021-05-30T20:43:57.752Z][INFO] GET '/packages/com.unity.formats.alembic@1.0.7' --> 200 (958 ms)
[2021-05-30T20:43:57.755Z][INFO] GET '/packages/com.unity.remote-config@1.4.0' <-- Request received
[2021-05-30T20:43:57.756Z][INFO] GET '/packages/com.unity.probuilder@4.5.0' --> 200 (947 ms)
[2021-05-30T20:43:57.770Z][INFO] GET '/packages/com.unity.scriptablebuildpipeline@1.15.2' <-- Request received
[2021-05-30T20:43:57.775Z][INFO] GET '/packages/com.unity.xr.openxr@1.1.1' <-- Request received
[2021-05-30T20:43:58.379Z][INFO] GET '/packages/com.unity.purchasing@3.1.0' --> 200 (812 ms)
[2021-05-30T20:43:58.387Z][INFO] PATCH '/project/dependencies' <-- Request received
[2021-05-30T20:43:58.404Z][INFO] PATCH '/project/dependencies' --> 200 (17 ms)
[2021-05-30T20:43:58.405Z][INFO] GET '/packages/com.unity.purchasing.udp@2.0.0' --> 200 (793 ms)
[2021-05-30T20:43:58.536Z][INFO] GET '/packages/com.unity.xr.openxr@1.1.1' --> 200 (761 ms)
[2021-05-30T20:43:58.818Z][INFO] GET '/packages/com.unity.burst@1.4.8' --> 200 (2044 ms)
[2021-05-30T20:43:58.820Z][INFO] POST '/project/dependencies' <-- Request received
[2021-05-30T20:43:58.849Z][INFO] GET '/packages/com.unity.cinemachine@2.6.5' --> 200 (2074 ms)
[2021-05-30T20:43:58.850Z][INFO] Resolving dependencies using SAT solver
[2021-05-30T20:43:59.902Z][INFO] POST '/project/dependencies' --> 200 (1081 ms)
[2021-05-30T20:44:00.912Z][INFO] GET '/project/registries' <-- Request received
[2021-05-30T20:44:00.916Z][INFO] GET '/project/registries' --> 200 (4 ms)
[2021-05-30T20:46:11.080Z][INFO] PATCH '/project/dependencies' <-- Request received
[2021-05-30T20:46:11.092Z][INFO] PATCH '/project/dependencies' --> 200 (12 ms)
[2021-05-30T20:46:12.094Z][INFO] POST '/project/dependencies' <-- Request received
[2021-05-30T20:46:12.130Z][INFO] Resolving dependencies using SAT solver
[2021-05-30T20:46:13.123Z][INFO] POST '/project/dependencies' --> 200 (1029 ms)
[2021-05-30T20:46:14.155Z][INFO] GET '/project/registries' <-- Request received
[2021-05-30T20:46:14.162Z][INFO] GET '/project/registries' --> 200 (7 ms)
[2021-05-30T20:48:41.273Z][INFO] GET '/packages' <-- Request received
[2021-05-30T20:48:41.277Z][INFO] GET '/project/dependencies' <-- Request received
[2021-05-30T20:48:41.359Z][INFO] Resolving dependencies using SAT solver
[2021-05-30T20:49:02.373Z][INFO] GET '/packages' <-- Request received
[2021-05-30T20:49:02.379Z][INFO] GET '/project/dependencies' <-- Request received
[2021-05-30T20:49:02.474Z][INFO] Resolving dependencies using SAT solver
[2021-05-30T20:54:45.434Z][INFO] PATCH '/project/dependencies' <-- Request received
[2021-05-30T20:54:45.456Z][INFO] PATCH '/project/dependencies' --> 200 (22 ms)
[2021-05-30T20:54:46.477Z][INFO] POST '/project/dependencies' <-- Request received
[2021-05-30T20:54:46.516Z][INFO] Resolving dependencies using SAT solver
[2021-05-30T20:54:47.727Z][INFO] POST '/project/dependencies' --> 200 (1250 ms)
[2021-05-30T20:54:48.770Z][INFO] GET '/project/registries' <-- Request received
[2021-05-30T20:54:48.776Z][INFO] GET '/project/registries' --> 200 (6 ms)
[2021-05-30T20:54:53.788Z][INFO] GET '/packages' <-- Request received
It’s just that these log files could potentially contain error messages related to your issue that may be helpful.
As for the ‘VPN or Proxy sentence’, it was merely a general hint as the Package Manager seems to have its problems with these things. Since you don’t use either of these, feel free to ignore that part 
New Problem 
In that case, I’d suggest doing a bug report so we can take a look at the log files and see what’s going on.