Q: MacBook 2012 is connected to wifi but won't load any web page. Says that the server could not be found. Every other device in my house works fine.

Information.

Safari 5.0.1 or later - Slow or partial webpage loading, or webpage cannot be found

Safari Problems

Also try another browser to make sure the problem is Safari.

Firefox

If you have any extentions loaded in Afari, try disabling them.

I have the same issue connected to wifi but I have some apps which needs to be installed but it cannot connect to the server and no answer to solve this issue

You might want to consider starting a new discussion. Since this one is marked solved, less people are likely to look at it. You can link to this one.

Try a restart.

Do a backup, using either Time Machine or a cloning program, to ensure files/data can be recovered. Two backups are better than one.

Try setting up another admin user account to see if the same problem continues. If Back-to-My Mac is selected in System Preferences, the Guest account will not work. The intent is to see if it is specific to one account or a system wide problem. This account can be deleted later.

Isolating an issue by using another user account

If the problem is still there, try booting into the Safe Mode using your normal account.  Disconnect all peripherals except those needed for the test. Shut down the computer and then power it back up after waiting 10 seconds. Immediately after hearing the startup chime, hold down the shift key and continue to hold it until the gray Apple icon and a progress bar appear. The boot up is significantly slower than normal. This will reset some caches, forces a directory check, and disables all startup and login items, among other things. When you reboot normally, the initial reboot may be slower than normal. If the system operates normally, there may be 3rd party applications which are causing a problem. Try deleting/disabling the third party applications after a restart by using the application un-installer. For each disable/delete, you will need to restart if you don't do them all at once.

Safe Mode

Safe Mode - Mavericks

Safe Mode - About

Safe Mode - Yosemite

Safe Mode - About El Capitan

Safe Mode Startup – El Capitan.