It may be because the operation steps are incorrect, you can refer to the following steps:

First enter the mobile desktop, find the Marriott Bonvoy APP, and click the Marriott Bonvoy APP icon

Enter the Marriott Bonvoy APP, select the corresponding login method, and log in to the account

After successfully logging into the Marriott Bonvoy APP, enter the homepage and click on the account to enter

Enter the account, find the settings symbol above, click enter

Enter the settings symbol, find the manage profile below and enter

Enter the manage profile, click add payment method below to enter, proceed Add to.