OH has a Samsung Galaxy A50 with Android v11 and the Tous AntiCovid app is v3.4.0 #310 and it all works ok.
Have you tried turning the phone off completely then turning it back on?
OH recalls there was a similar problem (wouldn’t update anything) using Google Play Store about a month or so ago and she fixed it by turning the phone off then back on again later which resolved it after Googling the problem and getting that advice as a possible solution.
I had to assist an older friend the other day when installing the Tous Anti Covid app on her phone. Google play would not install it and was complaining. I found the APK and installed it directly - it worked.
Below the two icons for the Apple store and Google play store, you will see a link to the APK
Worth a try before you end up doing a complete reset.
Thank you for replying Bettina.
His certificate did not have a QR code and his Amelie account had no information!
I spoke with Amelie this morning, the lady sent the certificate, with QR, printed and scanned to phone now and invalid message removed