You can absolutely return it for any reason assuming you haven't played it over a certain amount of time, steam has a very good return policy I have used it myself with no issue. PUBG wont run well on that system.
Once you buy a game from Steam you cannot get a refund for it after X amount of time. Only in the rarest occasions can that happen and it's only if the game was so bad that enough consumers complained. The most recent example of that was Batman Arkham Knight.
My advice to you is to do your homework before buying a game to ensure your hardware can run it. There are hundreds of links out there on PUBG player experiences and hardware requirements. With that said, with your hardware you will need to dial down quality settings and resolution to 720p just off the top of my head from years back.