Symptom
You run a migration with a PowerShell script that calls a list or library with -Name and get the following error:
Cannot validate argument on parameter 'List'. The argument is null. Provide a valid value for the argument, and then try running the command again.
Details
This error is common in PowerShell, and several things, including minor typos, can usually cause it.
If you are getting this error in a multilingual environment, it's possible that the name of the library in your script is not the same as its URL.
You'll usually see this with the Documents library, where you'll see Documents in the URL, while the library's name can be something like Dokumente in German.
You can open the list or library and verify its name directly in SharePoint if uncertain. Change the -Name parameter to the proper name; your script should run properly.