Verizon and Sprint use a different phone technology than other phone companies, called CDMA. To use a phone on those companies, you need a phone with built in support for their CDMA service. CDMA doesn't support removable SIM cards or changing carriers (or even changing phones, without having the carrier switch service to the new phone).
Some (many) phones on those services can also be used on the more common GSM phones services, because both Sprint and Verizon are new using that technology for data. That's where "unlocked" comes in. If you get the unlocked version of a CDMA phone you can use it on other (GSM) carriers. Sprint tend to only unlock phones for international use, not for use by their competitors. I'm less familiar with Verizon.
Either way, to use a phone on Sprint (or on Verizon) you generally need to buy a phone earmarked for that carrier. That is true of the Moto X.
If you can still return that phone, you should do so.