What do I need to do?

[Chrysamere]

Ok so I started to play not so long ago and I have a question. I want to continue on my way but before I want to continue, a message tells me that I don't have enough ADAM and that I didn't gathered a Little Sister. It tells me that the next area will be very tough to pass. As I am not a noob in shooters and since I chose the easiest difficulty, I think that I will be able to get through this section.
My question is: I want to save the Little Sisters so if I leave this one there will I be able to finish the game that way?

[Asherah]

You can leave behind a little sister and come back later if you want. The message is just to inform you that there is still a little sister wandering around the level. So you can leave and it won't affect the ending but if you want to get the acheivements than you have to deal with all the little sisters.

[Chrysamere]

I play on PC so I don't have the achievements. Thanks anyway!

[Asherah]

No problem, happy to help. :)