First open the "sshd_config" config file (/etc/ssh/sshd_config) en check these lines :
# Expect .ssh/authorized_keys2 to be disregarded by default in future.
AuthorizedKeysFile .ssh/authorized_keys .ssh/authorized_keys2
AuthorizedKeysFile .ssh/authorized_keys .ssh/authorized_keys2 ⇒ need to be uncommented
After that restart sshd service : systemctl restart sshd.service