Hi Sam,
Could you please check the BP number range in your system.
Your screenshot shows you have used Z1 as external number range. Please confirm Z1 has got alphaneuric range.
Also, you will have to enhance this exit and set the value as "BP" concantenated with the number and assign the new alpahneumeric number in to PARTNR_OUT.
Hope it would work. Let me know how you go.
Thanks
Bodhisattwa