grab does not work for screen capture. i click on grab over and over and nothing happens

Are you using the commands listed under capture in Grab's menubar? I am running OS10.13.1 and all the commands work for me. Here is Window capture(Command 5). All other options work also. As dwb says if you have a working keyboard the key commands are faster and easier.

There IS a need to use Grab, if you don't have your Mac keyboard. I have moved several times, and somewhere in one of the moves I lost the original Mac keyboard. No problem, or so I thought, because I have a solar-charging 3rd party keyboard. Sadly, the solar cells are dead, so I found an old PC keyboard and it works great EXCEPT for whenever I need the Command key. Until I open my wallet and buy another Mac-compatible keyboard, Grab is the tool for me. Except that it's not working for me either.

SO, even though the community helped solve Sean's problem, I still have mine, and I would appreciate an answer to Sean's original question!

There's no need to use Grab. Command Shift 3 performs a screen capture. Command Shift 4 performs a screen capture allowing the user to select the specific are of the screen to capture. The picture files are saved to the desktop. If this isn't happening, you must have changed the default keyboard shortcuts. Open System Preferences, select the Keyboard tab and look for the screen capture shortcuts. There are four - the two I mentioned save to the desktop. Two others perform the same task but save to the clipboard.

Take a look at the picture next to my post....granted I did travel with a carnival for a couple summers, but I was a ride jockey, not the bearded lady 😁