Hi Cyrus,
Thank you for sharing your question. I understand how annoying it is when you hit a snag trying to redeem your Microsoft 365 code, especially since you were planning to stack a few years. Usually, this happens because Microsoft 365 caps out at five years max. If adding the new code would push you over that five-year mark, the system won't let you redeem it. I am here to help.
To fix this, sign in to your account at account.microsoft.com/services and check when your subscription ends. If you're close to the five-year limit, you’ll have to wait until some time passes before you can redeem the new code. If you still get an error, even if you're under the limit, try redeeming the code at redeem.microsoft.com using another browser or device. Clearing your browser's cache might also help. If it’s still a no-go, reach out to Microsoft Support. They can double-check the code and what's going on with your account.
I hope this helps.
Best Regards,
Noel