redshift error when grant select on table: Operation not supported on external tables

You are seeing this error since SELECT is not a supported privilege type for external schema. Supported privilege are CREATE, ALTER, DROP. Refer following doc for details. https://docs.aws.amazon.com/redshift/latest/dg/r_GRANT.html

GRANT { { CREATE | ALTER | DROP }  [, ...] | ALL [ PRIVILEGES ] }
    ON EXTERNAL SCHEMA schema_name [, ...]
    TO { IAM_ROLE iam_role } [, ...] [ WITH GRANT OPTION ]   

If you want to grant SELECT privilege for tables in an external schema, use the following syntax.

GRANT { { SELECT | ALTER | DROP | DELETE | INSERT }  [, ...] | ALL [ PRIVILEGES ] }
    ON EXTERNAL TABLE schema_name.table_name [, ...]
    TO { { IAM_ROLE iam_role } [, ...] | PUBLIC } [ WITH GRANT OPTION ]