I struggled for hours until I figured out - if the list is named "Test" something, you cannot do anything. You can try to create the workflow, the window opens, but nothing works. No action adding, properties of workflow cannot be saved (dialog is displayed but not closed unless cancelled) etc. If the list is exported and imported with another name that does not start with "test" everything works.

Now why would anyone create the list named "Test" in the first place... well, let us start with the testproof or to be more precise "Testnachweis" in German for collecting the daily info about the corona tests for example...

Environment - SharePoint 2013, Windows 11, Edge in IE mode and Firefox - same behavior