SqlBulkCopy cannot access table

My issue was a bit different, turns out my table name was a reserved keyword in SQL so I had to do the following:

bulkCopy.DestinationTableName = $"{schema}.[{tableName}]";

Where schema is the target schema and tableName the target table name

From the documentation

DestinationTableName is a three-part name [database].[owningschema].[name]. You can qualify the table name with its database and owning schema if you choose. However, if the table name uses an underscore ("_") or any other special characters, you must escape the name using surrounding brackets as in ([database].[owningschema].[name_01])

Check that user that connects to db has

GRANT ALTER ON [dbo].[TABLE_XXX] TO [appuser] 

as suggested in answer by Jhilden on MSDN forum.