Failing to Install Monterey on a 2015 iMac

Hello, I am going through a series of travails in trying to install a more recent version of macos than Sierra on my external SD. I had initially installed sierra, but couldn't use apps. I tried to update to Monterey, but couldn't because the SD wasn't formatted to AFPS. I have now formatted the drive to AFPS and have my usb set up with the Moneterey installer, but I can't get the option to install from the USB. I just get the option to install Monterey from the web which always fails. Previously I was able to select my usb from start-up after holding option. Now it always brings me to the recovery mode menu. Has anyone else encoutered this? Thanks!