Your phone does have a SIM. It probably shipped with one already in it, and you just activated it without messing with it. All LTE phones need a SIM card, including Verizon LTE phones.
I honestly don't understand how or why Sprint SIM cards work the way they do. I can't say for certain if the SIM I linked will work in non-Apple devices, but I'd guess not. RingPlus sells like 8 different SIMs. I can take my Verizon SIM and put it in either my Moto X or iPhone SE, but the Sprint SIM card I got for my old iPhone 5s works in my iPhone SE but not my Moto X. There's a separate SIM card I need for my Moto X, which is what I bought tonight.
Your best option would be to find a phone you want to use on this plan first, then buy the proper SIM card. If buying a new phone right now isn't an option, then yeah, you might want to go ahead and order the SIM I linked above so you don't miss out on the next great promotion. You might have to buy a second SIM card depending on the phone you eventually buy for it, but at least you won't miss out on a good plan later on. This time they let you lock in on the promotion by buying something from their store during the promotion hours, but that's the first time I've seen them do that.
Another thing to keep in mind is whatever free plan you pick, you're stuck with it. I got the Phoenix plan (3000 minutes, 2750 SMS/MB) last week and missed out on two better deals (3250 minutes/texts/MB and the 4000 one today). There are a couple of ways to upgrade to better plans, but you either need multiple phones and lose your original top-up, or you have to downgrade to the Truly Free plan (awful) and wait a month.
Ah, okay. I actually Googled "phone credo sticks."
Well you do have to pay $2 I believe to swap devices. Really stupid, but that will just come out of the $29 top up you paid.