Bootcamp says "installation failed" after trying to download windows support software

I am sorry I don't have a definite answer for you, but in my experience, when an installation fails it is because the download was somehow corrupted. Can you put the file into the trash, empty it and try another download? Most often the reason is that the file has been downloaded via Wi Fi. Better to use an ethernet cable connected directly to your router/modem, even if you have to temporarily move the computer. If this is not possible, expect to do several downloads until you are able to get a clean one that works. And of course, that takes a lot of time for each download. I hope this is helpful and good luck.