How to fix error of Permission denied during doing SSH

I am trying to SSH to a new EC2 server from AWS by using Xshell, but got below error. Does it mean Public key in Client is not added into authorized_keys in Server side? How to verify which RCA for the error then give fixing?

[bastion-ro@ip-10-1-X-XX ~]$ ssh webuser@10.1.X.XX
Permission denied (publickey,gssapi-keyex,gssapi-with-mic).
[bastion-ro@ip-10-1-X-XX .ssh]$ cat /etc/redhat-release
Red Hat Enterprise Linux Server release 7.5 (Maipo)

Thanks for any advice.