You cannot change the open item management setting; (see long text)

What causes this issue?

This account does not have a balance of zero.

System Response

You <ZH>cannot</> activate/deactivate open item management with this
transaction.

How to fix this error?

If you want to activate open item management retroactively for an
account, create a new account with the correct setting and transfer the
existing items to this account.
IF &[SWITCH]S4H_DOCU_CLOUD& = 'X'.
ELSE.
IF &[SWITCH]FAGL_SFWS_UI_POSTING_SERV& = 'X'.
Alternatively, use the program <DS:RE.FAGL_SWITCH_TO_OPEN_ITEM>Switch to
Open Item Management</> (transaction FAGL_ACTIVATE_OP) to retroactively
set the indicator for open item management.
ENDIF.
ENDIF.
If you want to deactivate open item management retroactively for an
account, clear all the postings on this account first. Now you can
change the setting for open item management.

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.