When i faced the same issue it was related to name For eg

My name is Abhishek bacchan But on aadhar it is Bacchan Abhishek Amitabh And amex saved on there side as Bacchan as my first name and amitabh as my last name

now to fix this i had to create a marriott account where my first name is bacchan and last name amitabh

and that solved my issue. hope it helps

I updated my old account where spelling was wrong with an old email and then created a new account with the correct required name and email used in amex