If you're getting an error message that "This app is a free plan and should have a Bubble banner. Please upgrade to remove this banner. Removing or altering the banner in any way is against Bubble terms and can lead to your account termination." message when previewing the application/template," it is most likely due to a browser extension.
Other users have sometimes reported this when using a particular browser extension that affects the application (in particular, Dark Reader has been a culprit in the past). If you have this extension installed on your browser, we'd recommend disabling it.
If this isn't the reason in your case, you might try testing this after using the following steps to clear browsing data:
Delete your browser's cookies and cache using the browser's settings screen. It's very important to delete both, and you will be prompted to log back into most websites. If you are not prompted to log back into websites where you were previously logged in, this operation was likely not completed correctly.
Restart your browser by fully quitting the application and reopening the application. Closing the window or tab alone will not work.
Re-open the app's editor in a Private or Incognito browsing window and gauge whether the issue persists.
Remove browser extensions one by one.