Possible reasons for this error include, but are not limited to:
- Your ALLOCATE failed but you proceeded as if it worked
- Your ALLOCATE worked but you didn't save the CONVID
- Your ALLOCATE worked but the CONVID you used for the next command isn't the same as returned by ALLOCATE
- You never issued an ALLOCATE
- There was a network error and the CONVID is no longer valid
The quality of the response you get is related to the quality of the question you asked. You asked a broad, overly generic question with no details -- so you're going to get back a broad, overly generic answer with no details.