Hi,
For shop on behalf functionality, you can assign either User id in the format USXXXXX or specific organisational id O XXXXXX.
In case if you assign user id there shouldn't be any gap between US and id i.e. USxxxxxxx / for org id assignment I remember O space xxxxxxx.
You can try with other way if space doesn't work.
Please remember shop on behalf will work with either org id or user id I.e. SU01 id assigned in structure but not CP (central person) or S (position).
Regards,
Govardhan