Take a look at your grant settings for forms. Ensure that Manage Forms and Create Forms are enabled for non-admin users.

Hmm, I tested it and you actually don't need either of those grants to use forms in App Studio Apps.

I would submit a support case to Domo and let us know what they come back with.

Open the developer tools and get the message from the network tab when that toast pops up. That should help with diagnosing the issue. That or contact the support team and they can walk you through the troubleshooting.