You cannot change field &1

What causes this issue?

IF &[SWITCH]S4H_DOCU_ON_PREMISE& = 'X'.
In the Customizing activity <LS>Field Control for Prepayment</>, the
field &V1& is defined as unchangeable. Therefore you may not change this
field in the logistics invoice after a prepayment document has been
posted.
ELSE.
IF &[SWITCH]S4H_DOCU_CLOUD& = 'X'.
The field &V1& is defined as unchangeable. Therefore you may not change
this field in the logistics invoice after a prepayment document has been
posted.
ENDIF.
ENDIF.

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?

The fields &V1& may not be changed in the invoice that is currently
being processed.
Delete this invoice and enter it again.

Procedure for System Administrators

IF &[SWITCH]S4H_DOCU_ON_PREMISE& = 'X'.
You can check the settings in Customizing of <LS>Logistics Invoice
Verification</> under <LS>Incoming Invoice -> Prepayment -> Define Field
Control for Prepayment</>.
ENDIF.

Error message extract from SAP system. Copyright SAP SE.