Processing function missing for segment & (function & /&)

What causes this issue?

The parser is not familiar with any processing function for segment &
of the IDoc to be processed.

System Response

The system cannot process the IDoc. The documents in the SAP system are
not booked.

How to fix this error?

There may be a number of reasons for the problem. Check whether the
correct processing functions were assigned to the IDoc category via
definition of the workflow transaction. If the problem cannot be solved
this way, it is a program error. In this case, inform the SAP Hotline.

Error message extract from SAP system. Copyright SAP SE.