My rear brake rotor is lose

That is known as a "floating" rotor. It should move, but not that much. Definitely not front to back. Either the rotor hub and/or the axle is worn. There is supposed to be a boot and clip on both sides to keep dirt out..
Go to any one of the online parts sellers and look at a diagram of the rear be, you'll see what you're missing.

I see no problem with running it as is.
 

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Um it's lose and idk why it is any help? I'm I missing a nut or something???
It floats on the splined rear axle . It's a normal thing. Quit worrying and ride that Blaster! Worry when you hit the foot brake pedal and it keeps diggin! Then it's time to get excited.