I had a situation where my app worked correctly in browser, but failed to start in Appsheet on either iOS or Android. Error as subject and screenshot from mobile device as below. I contacted support who - after investigating - identified the issue was with the Firebase settings which I'd updated (some time back, so didn't make the link to the issue).
Info from Appsheet support: You have disabled the Allow Firebase toggle (found in Manage -> Integrations -> External Services). Apps with Firebase disabled can only run on a whitelabel (aka branded) native app or in the browser.
The error message doesn't help make this connection clear, so I hope sharing will allow others to resolve the same problem faster. Solved the issue immediately for me.