I'm trying to flash my grbl shield with firmware, but I can't compile anything as I keep getting an error compiling. I am using a 2010 Mac Pro computer with an Uno/Genuino board and a CNC V3 grbl shield. Initially, when I purchased an arduino package I was able to compile and execute small programs, but I can't even do the initial compiling right now. The program seems to be recognizing the correct serial port, I have the correct board listed but nothing works. When I've tried to move the motors with the Universal G-Code Sender I get an error code saying that grbl is still booting.
No idea about grbl or your shield.
Can you upload a simple sketch like blink to the Uno? If not, post the complete error message here (you can attach a text file to your post if the text is too big).
Is anything connected to pins 0 and 1 of the Arduino; that can interfere with the upload process. If the above failed, remove everything and try with the 'naked' Uno.
Which OS, which IDE version?