Can you paste the output or a screenshot of what you mean?
What do you mean “try to open”? editing the file on the CLI? Or some sort of GUI you’re using?
Either way this doesn’t sound like a restic problem but a permissions problem, but let’s see what you’re actually doing so we might be able to help diagnose further.
You may need to pass the --allow-other flag to mount if you’re browsing from a different system, or if your user doesn’t have permissions to access the data.