Ok so I did some debug as I still got the ftp accesses on your website.
It all went fine so I investigated a bit, to find that there:
https://sphynxlair.com/community/threads/hcm-awareness-bracelets.39933/
You have set the quantity to 0 !
From the beginning I was asking, have you set quantity to a value above 0 and you said me yes I did but in fact you did not !!!
So it's indeed the issue of the quantity when it reaches 0, but why did you answer you had set a quantity while you had not ?
Anyways you shouldn't be having the error anymore, I put a fix for the positive number issue.