You have created a Custom Field however even enabling its usage field is not contained in the Output Parameter Determination decision table.
SAP S/4HANA Cloud All versions
Reproducing the Issue
Business Application Example: Billing Document.
1.In app "Custom field and logic", create a custome field for billing document.
2.Enable the usage for output parameter determination.
3.Go to app "Output Parameter Determination".
4.Click the button Edit. Click table setting. Insert.
5.The custom field is not shown.
Data binding needs to be refreshed after custom field is added or deleted.
See chapter "22.214.171.124.2 Refresh Condition Parameters of Application" in the set-up guide of scope item 1LQ.
Output Parameter Determination, Custom field, Extensibility, Display, not available. , KBA , CA-GTF-OC , SAP S/4HANA output management , SD-BIL-GF-OC , Output Control Billing , Bug Filed