Someone from outside is trying to login our Fortigate VPN

How to prevent if someone trying to login Fortigate VPN, getting notification : Message meets Alert condition The following critical firewall event was detected: SSL VPN login fail. Every time remote IP is different. Our VPN has self signed certificate with strong credential. Because the person who is trying to get inside, from log showing error sslvpn_login_cert_checked_error. Any suggestion and advice highly appreciated.