Error code - 8016A0FA

I believe there is a 36 month maximum on the Gold card memberships. I had the same error message. I was told to wait until it gets to below 24 month and then try the card.

Sorry, I didn't receive any notifications to indicate that I'd gotten responses. The error code returned by the console is:

8016A0FA

What you are describing is the Error 8016A0FA and the OP hasn't reported that error code.

Yet I hope between us he/sh got the info required to solve the issue by now!

25 entries to what? Are you referring to an Xbox Gold subscription? If so, then the limit for stacking Xbox Gold subscription length is 36 months as noted in this support article:

https://support.xbox.com/en-US/billing/errors/error-code-8016a0fa

There seems to be some misinformation. You can indeed stack multiple subscriptions, for a maximum of 36 months worth of Gold at any one time. However, what could cause issues is if these aren't the same type of code: for instance, one of these is a 12-month
and another one is one of those promotional '12+1' month subscriptions, in which case they will not stack onto one another. So if that is the case here, then that would be the cause of the error.

Also, if adding 12 months would push you beyond 36 months, that would also cause this problem, though that does not appear to be the case here.

(Source for 36-month maximum:
support.xbox.com/.../error-code-8016a0fa)