The two most common reasons for not being able to add more user fields are:
1,000 Limit Reached:
A) How many user fields you have — go to Settings → Custom Fields to see your current fields (you can copy and paste into Excel to get a count).
The field name is reserved by the system or already in use
B) Is the name already in use or a reserved name? (To test this, just try adding a prefix — for example, if 'producttype' won't add, try 'amznproducttype' instead.)