It is in attack rolls and saving throws, but not for skill checks or ability checks, although, I do believe it is an existing optional rule...
Edit: Oh, I think I see where your confusion stem from. Yeah, had you been pushing the cart all by yourself, your check probably would have been high enough to push it aside enough to allow Roland's cart through, but the problem is you were performing an
Aid Another action to help Kayleigh push since she initiated a group effort to push the cart aside. Or at least, this is what I was thinking you were doing, you could have always "lonewolfed" it and try to push it all by yourself, ignoring the others' efforts...