Your domain availability check should be configure as follows:
Add the following as described in https://docs.whmcs.com/WHOIS_Servers

[
    {
        "extensions": ".ng,.com.ng,.i.ng,.org.ng,.net.ng,.edu.ng,.sch.ng,.gov.ng,.mobi.ng,.name.ng",
        "uri": "socket://whois.nic.net.ng",
        "available": "No Object Found"
    }
]

If you need further assistance kindly open a ticket.