There are a couple of reasons that could cause the error:
- the element was part of a generate copyback
- the use of generate with a nosource option
Delete the element in the From location - FEND stage P, and add it back into the same environment / system / subsystem / type allowed to move to the new mapping location.
It is recommended that if you change the mapping in an environment the best thing to do would be to empty the environment (migration, delete,archive or a retrieve/delete) prior to changing the mapping.