I dit a long struggle in finding the path for SDK and NDK but no possible outcome has appeared soon enough. If any of you guys know any solution, please be generous and put your solution or advice below in comments. Thanks in advance!
I will just recommend you to do as the warning says and download it through the Unity Hub.
Go to unity Hub, installs, then add modules and make sure you check the dropdown menu of the android build support, check the two options and download. This will make sure it is downloaded correctly and Unity should automatically register the paths for you.