I imported a citation from here http://www.archivesofpathology.org/doi/full/10.1043/1543-2165-133.4.542, and the URLs field is not editable
We tried on various accounts and could not replicate. Have you tried reloading Paperpile? Is the URL field always missing or was is just temporary?
If it’s still missing, can you select the item in the main list and hit Ctrl-j or (⌘-j on a Mac) and send us the debug output that is copied to the clipboard to support@paperpile.com.