Archintstall sometimes produces problems(at least I had problems with it). Make sure that you have the current iso version of arch on your stick and try again.
The problem I was facing was manually creating partitions. Should I use Gparted to make them first and then use archinstall, or does it not work with manual partitions?
It should work with both ways. First time I did them with archinstall(but didn't like that it created a separate partition for my home directory). Second time I manually partitioned my drive and then let archintstall use that.