The strap-on crash happens because the mod adds code that generates text based on the opposite side of the interaction. When applying strap-ons the action does not require any receivers, so that side will be empty and the new code will crash. The easy fix is to simply add someone to the receivers side of the action. Otherwise open up newsexsystem.gd and replace
var relation = globals.expansion.relatedCheck(cp, partner.person)
with(this can like 2 lines but it's just one line):
var relation = 'unrelated' if partner == null else globals.expansion.relatedCheck(cp, partner.person)
Basically it just adds a new if condition to prevent using a non-existent partner and adds 1 tab to each of those lines under it. The site converts leading tabs to spaces, so copying this text without converting them back to tabs will break it.
The human slave you attempted to give away was likely pregnant, thus triggered this error: https://itch.io/post/4914936
Edit: I failed to notice that the variable 'relation' was used in more locations, fixed the code replacement.
Edit final: Sorry, made a lot of mistakes on this one. I think I got this cleaned up now.