Creation of ETL Map only allows creation of new map only for import set tables are not being used.
Customer may have a lot of transform maps (in "sys_transform_map" table) already using existing data sources' import set table. ETL only allows to create new map if the data source's import set table is not being used.
Check existing transform maps in sys_transform_map table. Mark the map which is using import set table as inactive if customer wants to create ETL Mapping.