I am facing an issue with the QR code feature in Adobe Express. The “Manage QR” button is currently disabled, and I am unable to access or manage my QR codes.

First check you are signed in to the same account that created those QR codes, as a different account will not show them. If that is correct, sign out and back in, then open the design again and check Manage QR. If it is still greyed out, it is worth checking whether your codes were dynamic, since only those can be managed and edited after they are created.