logs in system event in FortiGate firewall about Admin login failed

This means that some program or human is trying to login to firewall from that particular server. 

1> Check which program or person trying login ? 

2> Verify if he is using correct credentials ? 

3> If you don't want to login to the firewall from that server, Add trusted host configuration under the admin users. 

For more details about trusted host and configuring security for admin users, visit the below link. 

https://community.fortinet.com/t5/FortiGate/Technical-Tip-System-administrator-best-practices/ta-p/189413