Error with assumed-role while trying to call a Lambda UDF from Redshift

I am trying to use an UDF Function in Redshift. and the IAM Role associated with it has "lambda:InvokeFunction" on all resource enabled.

However, when I tried to run the query :-

SELECT f_udf_func(person_name, 'A') FROM "users"."tbl_user_detl";

However, I am getting the following error :-

ERROR: User: arn:aws:sts:<account-id>:assumed-role/redsift-exec-role/RedshiftIamRoleSession is not authorised to perform : lambda:InvokeFunction on resource <lambda_arn> bec Detail:---------------------------------------

1. I am unable to figure out, since the IAM Role has all the necessary permissions for lambda.
2. Also, please tell me what this arn:aws:stsmeans . I had never come this previously