Hey,
normally when it's a code for a Stadia Pro subscription for 1 or 3 months, you would need to put in your card details. Don't ask me why, but it's the usual procedure. You can also check that out here in the list of Stadia offers, usually it's described in those T&Cs. Your card won't be charged btw, but would be if you don't cancel in time (if it's a free trial, then after the free trial ends, you would be charged).