In my case, when contacting my support person regarding the error message, she said she would have to activate my phone number specifically for app usage. As I understood, this would have been some sort of internal procedure (like adding my number to a database or some such).
However, I never went through with it all the way. While I wanted to test it, I prefer a PC for banking and deinstalled the app before finishing the setup.
But there are others in this forum who seem to actually using the ELBA app, they might be better suited to help you…