Username/PublicKey combination invalid error while loading data from EC2 to RedShift

While trying to load some data to Redshift from EC2 using COPY command, I am getting the below error.

ERROR: ErrorMessage: Username/PublicKey combination invalid ErrorCode: -18

I've added the public key of the Redshift cluster in the authorized_keys file of ec2-user. EC2 instance is running on AWS Linux2.

ssh_manifest file in S3 bucket is as below.

{ 
   "entries": [ 
     {"endpoint":"<public ip of ec2>", 
       "command": "cat test_users_pipe.txt",
       "mandatory":true, 
       "username": "ec2-user"}
    ] 
}

And below is the COPY command.

copy test2_users
from '<s3-bucket-uri>/ssh_manifest'
credentials 'aws_iam_role=<role_arn>'
delimited '|' region '<aws_region>'
ssh;