Once application can not validate, please provide a proper error message. The message on Validation page in the editor says:
The application is invalid for the following reason(s):
Application failed to validate.
Well, this is uselless. A low-code developer can not learn anything from it. It is necessary to engage a Domino admin to look into Domino log just to learn to look into error error-log-0.xml So you need a filesystem access to your Domino server :-( Further on, you need to enable traces in rcpinstall.properties, restart your Domino server :-( and browse the XML data in trace-log-0.xml
This is insane, please propagate the backend validation errors into the GUI so a developer gets some clue. Second, it would be nice to suggest some remedy steps.
(This might be related to server-side issues, refer to case CS0263206)