If you have already redem your redemption code you can't redem once again. On the first redem process you got an serial number. Take a look into your Adobe account to find the appropriate serial number.
If this doesn't help please described your problem more detailed.
What you done before the error occured?
on which step of the installation the error occur?
Thank you. Bought via Amazon in 2018. A one-off purchase with Product Key to redeem. Which was fine, till I tried to move it to this new PC. On the final step, after downloading and trying to open. The Adobe sign-in comes up, and asks for the input code, then it is just declined. I've tried to delete the programme and reload, and just it does it again.