Excessive document lines in FM document

What causes this issue?

The posting would produce a Funds Management document with over 999
document lines.

System Response

The posting cannot be carried out.

How to fix this error?

Check that the document does not exceed the maximum number of document
lines. Enter a smaller number of document lines or ensure that the FI
document is not summarized in greater detail than the Funds Management
document.

Procedure for System Administrators

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

Error message extract from SAP system. Copyright SAP SE.