There was no code for today’s game; you had to pay for it, leaving the space for the code blank as there wasn’t one.
Next Saturday, if you have a season ticket you should receive a code, so you should insert that instead of payment.
It’s either/or, not a combination of payment and promotional code. Next week it’s a code if you have a season ticket, or payment if you haven’t. Note the club’s advice about spam filters and e-mail addresses though.