A line already exist for these settings

What causes this issue?

The line you want to enter is not compatible with an existing line in
Customizing.

System Response

You can save two lines for the same document type in the same company
code: one for a posting key that supports the special G/L indicator, and
one for a posting key that does not support the special G/L indicator.

How to fix this error?

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

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.