Redeeming A Code for Xbox Game Pass

Our son got an Xbox for Christmas, and so far, we've had a heck of a time setting the thing up. It came with a code to redeem for one free month of Xbox Game Pass. Every time we go through the process of adding the code on the console, it says "parent
must set up recurring billing." So I go to my Microsoft account, and nowhere can I find anything about setting up recurring billing. Anyone? This is so frustrating. All he wants to do is play some games, and he's literally played the thing about five times
since Christmas because we keep bumping up against problems in setting it up. Thanks.

:)