Query to polybase external table giving error "Access to the remote server is denied because no login-mapping exists."
I don't know if this is directly related to your issue but the known limitations on PolyBase state that:

In order to use PolyBase, you must have sysadmin or CONTROL SERVER
level permissions on the database

As such you might try to:
GRANT CONTROL SERVER TO [LoginName]

However know that CONTROL SERVER gives you many rights on the instance such as the right to initiate a shut down.