My MacBookAir 2018 will not take screenshots. Why?

I understand you want to take a screenshot but it's not working. Did you follow these instructions: Take screenshots or screen recordings on Mac - Apple Support?

If the keyboard shortcut is not working for you:  (Apple menu) > System Preferences > Keyboard > Shortcuts > Screen Shots > Restore Defaults.

If you are using the "Dropbox" product I understand that it intercepts screenshots.

dawnlauriano wrote:

Shift, Command and 3, 4, and 5 do nothing.

Reinstall macOS according to Reinstall macOS - Apple Support.

If that does not correct the inability to take screenshots then contact Apple: Contact Support.

Needless to say, if you are using, or have used at any time in the past, any non-Apple "anti-virus", "cleaning", or "Internet security" products including "banking security", anything at all in that broad category of utterly useless garbage, don't. A lot of things won't work if you do.