There aren't any issues with the Mission Planner or the repository (I just tried on a range of boards), so this must be an issue with your PC. Have you loaded firmware with this board and PC before? If not, it looks like you don't have a driver installed.
the message is saying comport 8 doesnt exist. please make sure you pick a com port before clicking one of the firmwares.
I know this is a slightly old thread, but I am also now having this issue. Despite selecting a com port I cannot get the board to connect. I have tried reinstalling MP to no avail. This is on a system that has been working fine. I loaded the hex 2.8.1 firmware as well as upgraded to MP 1.2.25 and now cannot connect at all.