You wish to substitute GL account.
"Image/data in this KBA is from SAP internal systems, sample data, or demo systems. Any resemblance to real data is purely coincidental."
S/4 HANA Cloud
The substitution of the G/L Account is not supported in Cloud solution.
2676581 - FAQ: Validation and Substitution Cloud BAdIs Availability
2462725 - FAQ: Validation and Substitution for Journal Entries in S/4 HANA Cloud
Cloud, FIN_ACDOC_ITEM_SUBSTITUTION, FIN_ACDOC_SUBSTITUTION, F1481, Account, GLACCOUNT, G/L Account GL , KBA , FI-SL-VSR , Validation, Substitution and Rules , Problem