Probably need to Initialise drive, first. This is something that often needs to be done the very first time a brand new unformatted disk is connected to a PC.
it's been a while since I did it but IIRC right-click on the drive in the bottom-half of the screen and there should be an Initialise option. Select this. It should only take a few seconds and then the Format option will become active. Format will take probably take 1-5 hours depending on various factors