You created a new extension field of Global Data Type (GDT) using Key User Tools (KUT). Under GDT we have the following subtypes which can be used:
However when trying to use the new field on your PDI solution you're not able to as it doesn't show up in the references to Customer-Specific Fields file (.REF) in the Cloud Applications Studio.
SAP Cloud For Customer
GDT fields are not supported to be used in partner developed solutions.
This is the standard behavior of the system.
An alternative option may be to check the SAP Customer Influencer Site to submit an idea.
GDT, PDI, SDK, Partner Solution, Extension, Extensibility, Cloud Application Studio , KBA , BC-DWB-REP-EXT , Extensibility , How To