Approval number must be 24 characters long.

What causes this issue?

The approval number doesn't meet the 24-character length requirement.
It's a combination of multiple character strings, including the
3-character business code assigned by SmartBill. This error might be
caused by the following reasons:
IF &[SWITCH]S4H_DOCU_CLOUD& = 'X'.
The business code configured in your implementation of the BAdI
<LS>South Korea: Complete eDocument Data</> doesn't meet the 3-character
length requirement.
ELSE.
The business code configured in SAP Application Interface Framework
doesn't meet the 3-character length requirement.
Your VAT registration number configured in SAP Application Interface
Framework is incorrect. Therefore, the system can't fetch the business
code.
ENDIF.

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.