Hello, We understand how frustrating it is when apps are not working correctly. This article provides several troubleshooting steps to get it working again. https://www.samsung.com/us/support/troubleshooting/TSG01001496/
If you are still unable to log into the app, please reach out to the app developer for further assistance.
However, we do use Zelle using our banking app, and have never had an issue.
You might check your banking app for the Zelle option ... if there, simply ditch the Zelle app, as it would be redundant