my 2 day trial code won't work

I just bought a surface tablet and have been getting the same code for 2 days. This won't inspire me to use the paid version when my 30 day free trial is up if it won't work.

Sounds like you may have had gold previously and your account is past due or something. Check it out here https://account.microsoft.com/services If so you will need to contact support directly for assistance.
http://support.xbox.com/en-US/contact-us

Hope this helps. Good Luck!!

If you already were using payed gold membership before you can't use a trial membership. The purpose of trial memberships is to get to know the xbox live services at fullest not to extended the gold membership time.

To clear it up. If you have an existing 30 days subscription(trial or not). You should not be able to redeem a 14 day trial. If you where able to redeem a trial code during this period, then that 14 day trial will be useless as it won't stack with your 1
month subscription. Once your 1 month subscription ends, everything ends. It won't continue your trial subscription that you redeemed within those period.

So basically, if you have a free 2, 7 or 14 days trial. It is best to be used after an existing subscription ends. And make sure not to redeem them all at once. If you have redeemed a 14 days trial first then redeemed another 2 days trial, it won't stack.
It will overwrite the first one you redeemed. So instead of getting 16 days, you will only get 2 days of trial subscription.

I hope that clear things up regarding trial subscriptions.

Not sure about the achievement code, but if you're trying to use the 14 day trial on this account, it won't work because you're already gold. Trials cannot be added to gold subscriptions.