Let's say "You cannot edit this item here as it is checked out for editing in Catalog Builder" is displayed when viewing your app in Studio.

I would start by going to Catalog Builder.

And locate the app that is checkout out. In my example, the state was "in review". So I went ahead and selected Edit and followed the prompts to submit the app so that it was no longer in review.

Or if it is in draft, you can try to Discard draft.