You could not find a way to create special G/L indicator in a cloud system.
SAP S/4HANA Cloud 1902
Reproducing the Issue
You want to create special G/L indicator.
It is standard that special GL indicator cannot be customized via SSCUI. It is a pre-configured setting.
The changes in the basic setup of special GL indicators is non-approved expert configuration.
There is a SSCUI ID 100297 (Automatic Account determination) with which you can define alternative reconciliation accounts for special GL transactions.
Special G/L indicator, SSCUI 100297, create , KBA , FI-FIO-AR , Fiori UI for Accounts Receivable , How To