Zenmap only shows the router

Zenmap (GUI of nmap) is only showing the router and not showing any other device on the network, even that i am certain that there are other devices that is the code i entered nmap -T4 -F 192.168.1.1/51
and this is the output:
Starting Nmap 7.80 ( https://nmap.org ) at 2020-05-18 19:01 EDT
Illegal netmask in "192.168.1.1/51". Assuming /32 (one host)
Nmap scan report for 192.168.1.1
Host is up (0.0065s latency).
Not shown: 96 filtered ports
PORT STATE SERVICE
23/tcp open telnet
53/tcp open domain
80/tcp open http
443/tcp open https
MAC Address: 58:BA:D4:A4:D7:4F (Huawei Technologies)

I am new to kali, and i am getting used to it. My kernal version :5.6.0-kali1-amd64
Thank you.