Strongbox uses a 'Bookmark' to keep track of your database. This is the recommended Apple way of keeping a reference to a file on macOS. Most of the time this works very well but sometimes the bookmark can fail and you get an error telling you, you don't have permissions to view your database. This can also happen when you use a different version of Strongbox, such as when you upgrade from standalone macOS Strongbox to Universal Strongbox, or from Freemium Strongbox to Outright Pro.
Solution
The best thing to do in this case is to simply remove and re-add your database using the Databases Manager window.
NB: Before you do this make sure you know where you database currently resides on your file system, because you will need to re-locate it for Strongbox.
1. Go to the Database Manager window (View > Databases Manager)
2. Click once to select your database
3. Hit the delete or backspace key to remove the database from Strongbox's list.
4. Click 'Add Database' > 'Add from Files' or if appropriate Add from Google/Dropbox/OneDrive
That's it once you've relocated your database for Strongbox things will proceed as normal.
NB: If this keeps re-occurring, check that you only have one copy or version of Strongbox installed on your machine, because Strongbox's bookmarks are version specific, a database added on Strongbox Freemium won't work on Strongbox Universal/Outright Pro. This is an Apple system limitation. Often times you can end up running different versions of Strongbox by accident and you won't even notice it, but the bookmarks are not valid across versions.