How do you fix android build support?

I have every copy of the SDK installed but unity still refuses to allow me to assign it and always fails the build. Please how do I fix this I offer mobile ports of games to my other friends and this is severely hindering my ability to do so.

please help this is insanely confusing

What version of unity Are you using?
Because unity 2019 comes with its own Android SDK and NDK that are compatible with unity.
I also ran into a similar problem that you had on unity 2018, and so I upgraded to unity 2019 and installed the Android SDK and NDK that came with unity in the modules.
Then I ran into a grade build error problem and all I had to do was to connect to the internet and the problem was fixed and now I can build my own Android games