hi sean,
i’d like to confirm how you changed the admin role for the user. there are two places we can make this change, have you tried both?
first method:
1. go to office 365 admin center > users > active users.
2. select one user and then click edit user roles on the right.
second method:
1. go to office 365 admin center> users > active users.
2. select one user and then click edit on the right.
3. click roles.

if you still can’t assign the admin role, could you provide the following information:
1. could you try creating a new user in office 365 and test if you can assign admin roles to this new user?
2. could you provide a screenshot of the error message for our reference?
to upload screenshots, you can click use rich formatting at the bottom and then click the icon of
insert media.

thanks,
eric yang