Hey @1280397823, thanks for reaching out to the Community!
As stated in this support article, Spotify gift cards are not redeemable together with other discounts or promotions, such as Premium Family, Premium Students, or trial offers. This explains why you're getting that error message.
In that case, I'd recommend waiting a few days before your trial period ends to redeem your gift card here.
I hope this helps!
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Note: I'm not a Spotify employee.