You do not have permission to open the application

Greetings lunaNZ, 

It sounds as though you’re being prompted that you no longer have permission to a specific document. You’ve come to a great place for help! 

The following article provides great information on how to verify the permissions of the document in question, and how to change the permissions if needed.

Change permissions for files, folders, or disks on Mac

If the permissions are correct or the issue continues after changing the permissions, restart the Mac to see what happens. You can restart the Mac by clicking the Apple logo towards the top-left, followed by Restart in the drop-down menu. 

Let us know how that goes!

We’re glad you had the opportunity to double check the permissions lunaNZ, and were able to restart your Mac.

You mentioned this is happening with an app. To confirm, is this an application such as Word, Pages, or another app where you can create documents? If so, does this happen when trying to open different documents? If the issue is only with the app and not related to a specific document, check to confirm that Content & Privacy Restrictions isn’t enabled. The following article walks through how to verify those settings. 

Use Screen Time on your Mac

While these may not be features you’ve used, confirming they’re not currently enabled does help further isolate the issue. If you don’t have Content & Privacy Restrictions enabled, can you confirm if your user account is set up as an Admin account in System Preferences > Users & Groups?

If your account is set up as an Admin user, and Content & Privacy Restrictions is not enabled. The next step would be to contact Apple. You can find their contact details via the Get Support website.

Take care!