Possible...
But, can I make a suggestion? If you are not familiar with the platform, stop. Have someone who knows it help.
The buffer tube is held in place by a lock nut on the back of the receiver. You need the proper wrench to loosen it, which then allows you to back out (unscrew) the buffer tube.
I tend to think something else is up, I doubt Stag let that rifle leave with the buffer tube screwed so far in it prevents the upper/lower from going together, it was together when you got it right?
Cheers
Jay