Unable to include field in an InfoSet

What causes this issue?

The field you attempted to include in a functional group exceeds 999
characters.

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?

Define several additional fields with a total length of less than 1000
characters to hold the field you require. You can include these fields
in functional groups.

Error message extract from SAP system. Copyright SAP SE.