i tried to redeem a xbox gamepass code for 3 month free but it said You are not authorized...

It's frustrating, I and my brother had the same issue. I got him a 12 months code as a present for Gamepass Ultimate. The plan was to first pay only 1 GBP for the first 3 months and then redeem the 12 months code on top of the 3 months. But it didn't work.
We tried from both Xbox and Browser

If you got a 3 month subscription code for free, which is a 25 digit 5x5 code, then redeem it on the Xbox Store. There is an option in the store to redeem the code.

Hello there,

Yes absolutely! You can redeem any combination of gamepass/gold/ultimate code at any time. The only restriction regarding gamepass ultimate codes is you wont be able to redeem on top of the limit of 36 months or 3 years. Anything less than that, no restrictions.

I can't buy something i already own, discord gifted me a 3 month gamepass for free but it doesn't let me redeem it.

Hello!

Welcome to the Microsoft Answers Community, a community sourced forum for answers and discussions. Sorry to hear that you are having issues with your Xbox Gamepass code, here's some common issues with codes to check for.

1. Check that your code is 25 characters long.

2. You are redeeming your code at www.microsoft.com/redeem.

3. The code you purchased is for the wrong region. Codes can only be redeemed in the region they were purchased. If you purchased a second hand code online, there is a chance it was from a different region. Its always recommended to purchase codes from Microsoft directly or an authorized retail location.

4. It may be a Gamepass promotional code for new users only. If you used Gamepass previously either through a free trial, similar code, or previous purchase you may not be eligible to use the code. Check instructions carefully for conditions like this.

If you continue to have an issue with your code, you will need to contact Xbox Support for assistance. This page will showcase additional troubleshooting steps and a Contact Us button on the right side of the page for additional help.

Get help with a digital code or gift card (Xbox Support)

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Have a great day,