Trying to set up SSH Keys from Linux client to Linux sFTP server. Permission denied (publickey)

sFTP server - Oracle Linux 7

Linux client - Oracle Linux 7

I've created an sFTP server, first time doing this.

I think it works as I can log in to my sFTP server using the command $ sftp sftp_user@sftp_server_IP - and then enter the correct password, and I'm logged into sFTP.

I have ssh keys working between a Linux client and the Linux sFTP server. So, I can ssh between the client and sFTP server both ways without a password.

My problem is that I'm trying to connect from the client to the sFTP server without a password, and this doesn't work.