To verify if you have sufficient IDOCs available, use TCODE WE20 or WE02. Contact your SAP Basis team for details on the IDOC limit for your environment.
To verify if the Target Logical System is already defined, check the /QTQVC/SRCLOGSYS table.
To verify if the IDOC was created at all, check the RSBASIDOC table.
Cause
Two likely root causes:
The SAP environment has run out of allocated IDOCs (Intermediate Docs)
A Logical Target System with the same name already exists