SSH connection denied by EC2 instance

I'm trying to SSH into my instance on EC2 but getting permission denied. Port 22 is open in the security group. I set 400 as a permission for the file which is setting that supposedly works in this case as I need only read. I can set 600 but in this case it's unnecessary.

Where should I take a look ? How can I fix this ?

ec2-user@35.178.89.23: Permission denied (publickey,gssapi-keyex,gssapi-with-mic).

verbose flag revealed:

debug1: Host '3.8.3.177' is known and matches the ECDSA host key.debug1: Found key in /home/mark/.ssh/known_hosts:1debug1: rekey after 134217728 blocksdebug1: SSH2_MSG_NEWKEYS sentdebug1: expecting SSH2_MSG_NEWKEYSdebug1: SSH2_MSG_NEWKEYS receiveddebug1: rekey after 134217728 blocksdebug1: SSH2_MSG_EXT_INFO receiveddebug1: kex_input_ext_info: server-sig-algs=<rsa-sha2-256,rsa-sha2-512>debug1: SSH2_MSG_SERVICE_ACCEPT receiveddebug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-micdebug1: Next authentication method: gssapi-keyexdebug1: No valid Key exchange contextdebug1: Next authentication method: gssapi-with-micdebug1: Unspecified GSS failure.  Minor code may provide more informationNo Kerberos credentials available (default cache: FILE:/tmp/krb5cc_1000)debug1: Unspecified GSS failure.  Minor code may provide more informationNo Kerberos credentials available (default cache: FILE:/tmp/krb5cc_1000)debug1: Next authentication method: publickeydebug1: Offering public key: RSA SHA256:rSHi+GMCUIvsF4HA0Kv5C0++sLGGiquVQ/z/OXQBPEU mark.alexa@mail.comdebug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-micdebug1: Trying private key: /home/mark/.ssh/ecs.pemdebug1: Authentications that can continue: publickey,gssapi-keyex,gssapi-with-micdebug1: No more authentication methods to try.ec2-user@3.8.3.177: Permission denied (publickey,gssapi-keyex,gssapi-with-mic).