Apparently the Ebay store creation is totally broken in sandbox. If I go in with my sandbox account, I was able to convert it to a business listing after much issue, but all the links to create a store are broken or add store subscription is broken as well. Haven’t found any other way to get to stores or items, their own dev guides suggest these ways but they are both broken. Seems like these issues have been happening unresolved for years. Googling this topic shows years of this features non-working status.