When trying to make the pack I get the error "Can't detect .ogg file length while converting." Any ideas?

Which version do you have? This is sometimes caused by OGG files with corrupt headers. I would remove tracks one by one to figure out which file is the problem, then run it through an online converter like https://audio.online-convert.com/convert-to-ogg.

If you're on the latest version (v1.6.2) you can also try checking the "convert .ogg files instead of copying" setting.