Gary,
I have a machine I designed and built myself using steel box section lengths I scrounged of the local tip and all the clever bits bought on Ebay and AliExpress. I'm using LinuxCNC running on an old Windows XP era PC so cannot advise you regarding Grbl, though if you haven't specifically, and sucessfully homed the machine by some means before starting to try and cut anything this may well be the problem.
Getting a machine up and running can be frustrating at first, there are so many things you didn't know you didn't know, but once it's sorted you'll have a lot of fun finding new things to make with it.

Kit