Been running into this frequently over the last couple weeks where the Package Manager Console commands for Entity Framework Core stop working and you get the infamous "not recoginized as the name of a cmdlet, function, script file, or operable program" error message. This problem has been a more or less continual plague in Visual Studio and Entity Framework (both original and Core versions) for years now.
At some point you would think someone on either team would finally get this working right, but it somehow keeps cropping up. In this particular case, neither re-installing nor restoring the nuget packages worked for me. I finally had to resort to running a powershell script found in the EntityFrameworkCore.Tools nuget folder.
For those experiencing this problem, you have to run the following command from within the Package Manager Console (not the command prompt or Powershell command prompt).
This will get the Package Manger Console working again for Add-Migration, Update-Database, etc. That is, until you close Visual Studio anyway. Most of the time, you have to re-run the command next time you run Visual Studio.