I have 3 months of Apple TV from a purchase but I can’t redeem it

What purchase was that? Not all Apple products have this offer.

If you have had the service before, then you may not be eligible for new free trial. One trial per customer.

If you waited too long to redeem, then the offer may not be available anymore. The offer should be redeemed within the first 90 days.

If the above doesn’t apply to your situation:

Redeem the Apple offer on your new eligible Apple device in the TV app, not on a different device.

Redeem offers from partnering companies in the manner that they indicate.

Read here how to redeem: Apple TV+ offer - Apple

Read here about the conditions: Apple TV+ offer - Terms and conditions - Apple

Read here if you have trouble redeeming: If you can't redeem your promotion for an Apple subscription - Apple Support

↳ Step 5 says to contact Apple if the prior steps didn’t bring a solution. They can assist.

Note that for Macs, users have to additionally sign in to the TV app (menu Account), for the redeem option to appear.