Reboot the computer (without reopening current windows), then look in:
~/Library/Application Support/Audacity/Session Data
where “~/” is your home folder.
Find the file:
audacity-lock-<yourUserName>
and delete it.
You should now be able to start Audacity.
When shutting down the computer, ensure that Audacity is completely closed before you shut down (by default, macOS keeps applications running in the background even when all of the applications windows have been closed).