why not internet sir?

You aren't speaking to Apple here. You aren't speaking to your internet service provider.

However, Apple menu -> System Preferences -> network will yield clues as to the kind of internet you may have.

If you have a modem that supports etherent:

And your Mac can connect this cable into a port like this:

A similar port is available on many USB-C dongles for ethernet access. Make sure when you hook the cable into the port it clicks into place.

Connect the ethernet cable to the port that looks like this, so you get ethernet in Apple menu -> System Preferences -> Network. It may say Thunderbolt ethernet if you are using an adapter for ethernet. USB 3.0 and earlier ethernet adapters may not work.

Note the IP address will be your clue what is happening, as well as the color next to the port on the left hand side.

10.x.x.x addresses and 192.x.x.x are typical addresses for both WiFi and ethernet routers that route traffic from the modem itself.

169.x.x.x address indicates something is wrong with the internet connection of the modem, and usually will be compounded by a yellow dot next to it. Sometimes changing the location to automatic helps. Sometimes going into advanced and removing any VPN or proxies will help, but only if you aren't on a corporate/school network. Corporate/school networks need VPNs and proxies, and those configurations will need to be determined by the system administrator of your home office.

If you don't have an ethernet connection, a WiFi connection is likely, and you'll want to make sure after selecting WiFi that the status is checked off in system preferences as it is below:

Once it is, check that the WiFi menu in the upper right, is not looking like:

If it is, then you'll need to turn on the WiFi network, and connect to the appropriate WiFi for your location.

Most WiFi routers from internet service providers have the network name and password written on one side of the modem.

And usually they are connected by WPA2 Personal networks. In some corporate/school environments, you need to "join other network" and select WPA2 Enterprise. Those environments when you move between rooms, may need to be reconnected that way, as the modem may not have registered its security with your computer. If you are still having trouble, contact your internet service provider.