You don't have any sort of a free trial attached to your account. If you were offered a 30 day trial on the console dashboard you'll need to add a payment method in order to take advantage of it.
If you redeemed a trial code like a 2 day or 14 day trial, or even a 30 day trial code from a redemption card then you wouldn't need any sort of payment details. The 2 day/14 day card doesn't give you gold status but lets you play online for the stated days
(2 or 14 days, depending on what you redeemed), or 30 days (which will give you gold status) if you redeemed it from a gold subscription card.
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