i think the issue is that you are trying to create a DELTA table in Unity catalog from an Parquet source without converting it to Delta format first.

As Unity catalog will not allow delta table to be created in an non-empty location. Since you want to expose directly in UC without writting delta log, you can try by creating external table
%sqlCREATE EXTERNAL TABLE catalog_name.schema_name.table_name

USING PARQUET

LOCATION 'S3 location
but you will lose Versioning and time travel. 

Hope this will work otherwise 
convert parquet into delta in place first then try to register external table. But i am sure i have done directly exposing in catalog by simply registering external table 

@Jennifer