I’d like to use an old key that I already deleted.

[Panicha P_]

I’d like to use an old key that I already deleted, because the first one was created using the wrong template, and I want to create a new one.

[Nikola Perisic]

Welcome @Panicha P_ 

You can reuse the old project key, when you delete the project from trash.

[Tomislav Tobijas]

Hi @Panicha P_ ,

Have you completely deleted the project/space or just sent it to trash? If you've just sent it to trash, you will need to delete it permanently to reuse the space key.

If you've only updated the key on the existing space, unfortunately, there's no way to reuse the old key, and you'll have to delete the space first to reuse the old value. Jira app does support this, but JSM currently does not.
There's a feature request for it, though: JSDCLOUD-15235: Enable releasing a project key for JSM projects 

Hope this helps.

Cheers,
Tobi