I tried to bypass factory reset protection on a Galaxy S4 but it didn't work I've been trying to bypass the Google Account credentials on a Samsung S4 I was given (it had already been factory reset) and, following the instructions in this answer, I inserted a USB drive via the OTG at the Google Account notification - but nothing happens! ie, no, popup or new page, just the annoying 'This device was reset etc' page and the unhelpful option to enter an email.

What can I do now to access the APK from RootJunky?
There are plenty of tutorials on YouTube on how to bypass the FRP. It's quite easy on S4 though. You can't see the USB OTG just because it is not enabled on your phone. Try the keyboard hack to bring the browser or bring the e-mail app. I can guide you form there.