Open the side plate that has the magnetic controls. You can easily access the spool & make sure nothing is caught in there.

My guess is like Dave's - either your spool tensioner knob (the black knob by the handle) is set too tight, and/or you have all of the mag cast controls set (you push them "in" to release, pull them "out" to get more cast control). Try loosening the spool tensioner, releasing all the mag cast controls, & loosening the star drag some. Then, if it frees up the spool, try messing with just one of those things until you figure out what's wrong.

If all else fails, take it back to where you bought it & exchange it. Shimano makes a great product & it would be rare to run into a defective one. But, it could happen.