Hi,
May I know why you want to add the customer table as reference table and field in the characteristic? Since you are using material class, you can very well proceed without the reference table also. You can maintain the reference table and field as blank in CT04 (as shown in my previous screenshot) and proceed.
If you still think the customer master reference is necessary, necessary, then refer the note: 75899 - Ref. characteristics wrongly/not processed
Regards,
AKPT