I have the same problem here. I've been getting that "Loading Failed..." message since 18 beta came out. I assume the idea is to let you choose which beta program you would like to load a beta for – same as with all the other devices, including tvOS. This option works fine with all the other apps. When 17.6 came out, so far I've updated iOS, iPadOS, and tvOS successfully. But no go for HomePod OS (on iPhone). I get the "HomePod Beta Uptates" menu (under Updates in Home app settings), but never get past the "Loading Failed" error. I've also tried it using my iPad. Same problem. I have also reset the HomePod to factory 17.5 using the Mac, USB connection, and Finder. When I first went to update to HomePod 17.6, I did have the option to update to HomePod 18 above the HomePod Beta Updates menu. I'm not quite ready for 18, so that's when I tried the HomePod Beta Updates menu. After resetting to factory, I no longer have that option, only the HomePod Beta Updates menu. Help!