Congrats about your new rifle !!

Seems like upper and lower fitting is too tight. Well, it will be much more loose after couple years of hard use...
Bring it first to someone who knows AR-15 and can tell you it is not binding so much that lower receiver would break during use.

You propably used gunoil to lubricate pivot pins ?
If oil does help but not enough, and it is very close whether you can install pivot pins by fingers, you can test the following : take pivot pins out and polish them with 1000 mesh sandpaper or similar device by hand.
Do not use Dremel, you want to keep those pins round

Used to fix my first own machined upper / lower lot that way and the end result was nearly perfect