Hi Pal,
If you see this error even after finding your customer in GTS, you will have to check table /sapsll/pntbp.
There might be inconsistencies in this table See if you find the same BP# in this table for your customer. BP# and External partner# mapping is maintained in this table. GTS will identify the BP# in GTS with the external partner# using this table.
If you see a different BP# mapped to your customer in GTS, please use BUPA_DEL and the pointer table cleanup program /SAPSLL/BP_DATA_CLEANUP and then re-transfer your customer from ECC to GTS.
Regards
Dhilipan