Unity Project doesn't have solution file

If "Open C# Project" doesnt create the .sln file, try updating your external tools first.

In Unity, go to Edit > Preferences, and make sure that Visual Studio is selected as your preferred external editor.

This created the sln file for me.

I finally found the answer.

I closed Xamarin and inside Unity went Assets > Sync MonoDevelop Project Doing this created the two .sln projects: -csharp.sln and .sln