unity3d 5.5.0fx Unable to list target platforms error

That seems related to the facebook SDK and some dlls it brings in. I deleted them and everything works.

 

I did the downgrade, but now getting an error I need to upgrade my SDK. I clicked Used Highest Installed - and now I'm getting an error saying Require API level 26.

 

The build tools being suggested are ridiculously outdated and a solution is unlikely.

https://mega.nz/#!s9RCDY7D!j5_xU7UUsyH6jxIbwxNQweuKsrRhD7Rxl_w-TXcYhb8
SDK Tools, Revision 25.2.5 (January 2017)

Find your SDK path (Android Studio -> Tools -> SDK Manager -> Android SDK Location)
Duplicate the entire SDK folder, extract the downloaded zip, replace the tools folder
Direct Unity to use the tools you just duplicated (Preferences -> External Tools -> SDK)

Warning: This workaround assumes you also use Android Studio for development
The SDK folder can be over 50GB. Simply replace the tools if you only use Unity.