When I open the app I am presented with the screen shown in image 1.
I enter the 13 digit number and password and get screen 2.
Then I select Health Certificate and get this screen.
Then I select the Vaccination of Covid-19 and it displays the vaccination certificate.
Have you got the latest version of the app?
My version says 1.3.2 (303), store 1.2.8, Production mode.
Sorry about the crap images, but the app security prevents screen copies from being taken and I had to use my wife's phone to take pictures.