If a user can't access your app, there are three possible reasons for the problem:
The app itself is not runnable: When there's an error in the app definition, the user will see a message indicating the app isn't runnable, with direction to contact the app creator. As the app creator, you should open the app in the app editor and check the Info tab for error messages and resolve any you find.
The user doesn't have permissions to access the app: this occurs when you have required user authentication for your app and the user's email doesn't match any entry in the user allow list for the app. The email address associated with the user's sign-in account must match the email address in the allow list.
One or more tables in the app require "as app user" access: this occurs when there's a security option you may have explicitly enabled. If you do so, the user's account must have permissions to access the underlying data for the table (typically, a spreadsheet in cloud storage). Often, app creators invite a user to try their app but forget to give the user appropriate permissions. For this reason, we recommend the "as app user" security access option be used sparingly and only when absolutely necessary.