Iirc the "sandbox" of itch app consists of the itch app creating a new window user. If that user already exists from a previous attempt, there might be errors. Or if you somehow modified that user account after it was created.
To create that user, the itch app obviously needs some privilege escalation to admin rights, but I do not remember how that was done or asked for.
So if you have downloaded the itch app from itch directly, maybe try starting it once as an admin to see if you can configure sandbox.