Active Directory Connector cannot connect but the LDAP is connected is successfully

[SFW]

FortiGate-81F # diagnose debug fsso-polling detail
AD Server Status(err: server can not be accessible):
ID=1, name(172.18.0.1),ip=172.18.0.1, port=0, source(security), users(IPv4:0, IPv6:0),
username=swd\lcloperator2
read log eof=0, latest logon timestamp: Thu Jan 1 03:00:00 1970

polling frequency: every 10 second(s), success(0), fail(106)
LDAP status: init

LDAP query: success(0), fail(0)
LDAP max group query period(seconds): 0


this is branch location firewall the AD is in DC location 
also i checked the Fortinet documents but still i didn't find any solution 
can you please help me on this 

Note: Agentless polling mode

[AEK]

Do you confirm in LDAP server config the "Test User Credentials" works successfully ?

[SFW]

Tested Successfully

[AEK]

Can you check if 172.18.0.1 allows FGT to connect to port 445 TCP?

You can try a telnet test from FGT to DC:445.