Entry for no. of IDocs is missing. IDoc written to file.

What causes this issue?

You have chosen the option of creating an IDoc inbound file, but have
not stated the number of IDocs (repetition factor).
A single IDoc is written to the file.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Error message extract from SAP system. Copyright SAP SE.