When you try to apply a security policy ona PolyBase external table in SQL Server 2019, you may encounter an error message that resembles "Security predicates can only be added to user tables and schema bound views". With this fix, you can create a security policy on PolyBase external table and use Row-Level Security (RLS).
Note The fix will be disabled by default, and you should enable the trace flag (TF) 4043 before applying RLS on an external table.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.
This issue is fixed in the following cumulative update for SQL Server:
About cumulative updates for SQL Server:
Each new cumulative update for SQL Server contains all the hotfixes and all the security fixes that were included with the previous cumulative update. Check out the latest cumulative updates for SQL Server: