When FortiGate's LDAP authentication can't find any matching DN it reports "get all dn - found no DN," which usually means the user either isn't in the group(s) you've told FortiGate to check or the account isn't currently active.
Ensure the user is a member of at least one AD group.
If you've configured FortiGate to restrict logins to specific groups, any user not in one of those groups will simply never be found by the LDAP search.
Ensure the account is active.
FortiGate's default search filter excludes disabled or locked accounts, so a disabled user object will likewise never show up in the DN search.