Account operation is only allowed in target account field

What causes this issue?

You can carry out specific modifications to the account keys of the
source selection during the copying process. The rules for this
modification are stored in the G/L account field of the target
selection.
o For example, if you enter '++XX' or '++XX*', positions three and
four of all selected account keys will be replaced by the
characters 'XX'
o With I/<start_position>/<insert_string> it is possible to insert
characters in the selected account keys at a certain position.
For example use 'I/03/W1' to insert the string 'W1' at position
three in all selected account keys.
o With D/<start_position>/<length_del_string>, it is possible to delete
the characters occurring at a certain position from the selected
account keys.
For example, use 'D/03/02' to deleted two characters - starting from
position three - from all selected account keys.

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?

Please define the account operation in the G/L account field of the
target selection.

Error message extract from SAP system. Copyright SAP SE.