The valid theme park admission associated with your Site/App account and used to make FastPass+ selections must be the same valid theme park admission that you will use for entry into the park on the day the FastPass+ selections are redeemed.
This reportedly comes after third party "tour guides" have been abusing the previous system with extra Magic Bands that are not linked to a My Disney Experience account. These tour guides would make Fastpass+ reservations at the kiosks to be used for guests that hired them as "guides".
Accounts may be locked if a guest has redeemed a Fastpass+ selection on a ticket that was not used to enter the park on a particular day. Accounts can typically be unlocked at guest relations.