Unable to access Websites on my Mac mini via Wifi

Is the Mac mini connected to Wi-Fi?

Unless its been connected before to a Wi-Fi network automatically, it won't do so one its own.

Click on the Wi-Fi icon on the menu bar and make sure a Wi-fi network is selected and actively connected.

Connect to the internet with your Mac - Apple Support

Make a New Location, Using network locations in Mac OS X ...

System Settings>Network, top of window>Locations>Edit Locations, little plus icon, give it a name.

1. On your Mac, choose Apple menu  > System Settings, then click Network  in the sidebar. (You may need to scroll down.)
2. 
   
3. Click the Action pop-up menu  on the right, then choose Set Service Order.
4. Drag services into the order you want.
5. Click OK.

The interface that connects to the Internet should be dragged to the top of the list.

System Settings>Network>choose interface>Advanced>Proxies Tab, make sure none are set, like for HTTP & HTTPS.

System Setttings>Network, click a network service on the right, click Details, then click DNS. (You may need to scroll down.)

1. On your Mac, choose Apple menu  > System Settings, then click Network  in the sidebar. (You may need to scroll down.)
2. Click the network service you want to use (such as Wi-Fi or Ethernet) on the right, then click Details.
3. Click DNS, click the Add button  at the bottom of the DNS Servers list, then enter the IPv4 or IPv6 address for the DNS server.
4. Click the Add button  at the bottom of the Search Domain list, then enter the search domain.
5. When you’ve finished, click OK.

Enter these numbers...

8.8.8.8

8.8.4.4

1.1.1.1

9.9.9.9

Apply.

Good point,:)

If using Wifi, instead of joining your Network from the list, click the WiFi icon at the top, and click join other network. Fill in everything as needed.

Good work, take care my friend.:)

is the wifi activated?

you will likely also need to enter password (in the case you have never done so…) Sorry if that sounds basic, but you did not provide much details. Obviously you don’t have internet connexion now.

Wifi is not like ethernet which usually connect you direct and straightforward.

Do a diagnostic :

On your Mac, quit all open apps. Try to join the Wi-Fi network you're having problems with (if you're not already connected). Press and hold the Option key, click the Wi-Fi status icon in the menu bar, then choose Open Wireless Diagnostics. Follow the onscreen instructions to analyze your network connection.