Last night, I finished my Unity work. I saved it, closed it, then reopened it and build it, then saved and closed it again, then reopened the build and it worked fine.
Today I cannot find the build anywhere on my computer and it is now coming up with this error message:
ApplicationException: Unable to find a suitable compiler
UnityEditor.Scripting.ScriptCompilers.CreateCompilerInstance (MonoIsland island) (E:/BuildAgent/work/71ca6fec1b41cc30/Editor/Mono/Scripting/ScriptCompilers.cs.101)
I had some students in my class run into this error as well. What I found was they were creating published builds and saving them inside of the Assets directory. By simply deleting the builds or moving them out of Assets, the errors went away!