I also had a problem. It seems there is a “quirk” in the system. I did not get a certificate from the Test Centre but downloaded it from https://attestation-vaccin.ameli.fr/ and had to login through France Connect. I then saved the pdf and printed it. However, if you already have the Certificate with QR code from your Dr, you shouldn’t need to do that.
I found that scanning it through the TousAntiCovid App / My wallet produced the error you have posted - in the wrong format. I eventually managed to Add the Certificate by passing the phone camera (not through the App) over the QR code (on the right hand side of the certificate) and it came up with a website message and asks to Open it with AntiCovid. I did that and it asked if I wanted to add this Certificate to My Wallet. I clicked on “Add” and voila - there it was in my Wallet! (Although another strange thing - it seems to show it as the 2D-DOC which in all the help files it says not to use!) Still, all the correct information is shown, so I don’t worry about that!
Using Version 3.1.7 #260 on my Samsung A50 android phone.
We tried using Graham’s App on his phone with Version 3.1.8 #262 and it seemed to scan it in properly through the Add Certificate in My Wallet.