macOS error: The application is not open anymore
I encounter this regularly. Usually I can track it down to the application immediately crashing - but Finder/the taskbar do not get notified and no way to remedy.
Unfortunately there is no easy fix.
What usually works is:

In most cases I encounter it is solved by deleting/reinstalling the app.
Quite often it helps to reboot and try again.

I know - both approaches are very disappointing for 2021.
Two rather sophisticated tips - that require some understanding and may leave you with a system that will not work without reboot(!) - and unfortunately rarely help are:

You can try to use &quot;Activity Monitor&quot; to assure the app and related processes are really shut down, and try again after this.
to launch an app from command line works  more often - but if it works it will leave you with a second symbol for the app in the dock - or in case of error might give a little more info what is going on.