I had the same error when I updated unity from version 2018.3.11f1 to 2019.1.0f2 and I tried all the fixed I could find on the internet, what SOLVED it is just by deleting “Drive:\UnityProjects\Project_Name\Packages\manifest.json” file and restarting unity.
Try Reimporting assets from “Unity:-Assets->ReimportAll” see if it works for you.
I encounter this issue switching from 2018.3.f1 to 2019.3.f1 (Linux)
I tried:
It’s added to the hundred bugs of Unity Editor 
This fixed the issue I was having in Unity 2019.12f1 upgrading my Vuforia samples.
Thank you! Solved!!
thank you problem is solved
thank you this solved my problem , many thanks .
Thank you, Karl!
Hey this thread is a little old, but what solved this for me is when I “upgraded” my unity and opened my old project, Unity put some of the old unity engine tools in a folder inside my asset folder. It was named 2d-extras-2018.3. I deleted this folder, then some assets auto imported, then my build errors were fixed
Great ! Solved my Issue. Try this if you are still facing same problem
THANKZ DUDE!!!
Just update your package in package manager
THIS MAKES EVERYTHING ERROR ,
I was trying to complete Junior Pathway Programmer and it required version 2018.4, but I tried it on 2019.4 and it gave me the CS0246 errors. This solution worked for me very well! Thank you @karl_jones
Thank you so much, this worked!
Also voting for the Help / Reset packages, worked great. Just had to re-install ProBuilder (was wondering why some of my textures were pink…).
It worked perfectly for me. Thanks.
I just upgrade package Manager UI from package Manager all errors gone…
I’ve just removed the package and everything worked as usual
Saved my day, Thank you!
Go into package manager → in-project → package manager UI and remove it!