I struggled with this a few months ago. I finally contacted Celebrity (via Twitter) to ask why my 2 attempts at uploading the Covid Pass was not accepted. They looked into it for me and explained the first one I uploaded was 'a bit fuzzy', so presumably I had moved when taking the photo, and my second attempt had glare from a screen I was taking the photo from. It was suggested I print out the Covid Pass and upload a photo of that. It was then accepted, and I had the printed copy to show at the port if necessary.
Good Luck.