What do girls mean when they say that it's nice of you to like them, but that you can't be together?

Either she really can't be with you (personal reasons, parents, another guy, etc.), or thought it was sweet that you like her, but she was letting you down easy because she doesn't like you in the same way. I'm sorry.

They are flattered, and think it was sweet, but they don't like you back. It's a nice way to say no while keeping your pre-existing friendship (if applicable) intact.

She's not into you bro, sorry :/ Just don't be a sucker for love like my best friend is with this one girl, he liked her, she didn't like him back, she realized he really likes her, now she takes advantage of him and plays him like a puppet on a string.

She's not interested and doesn't wanna be a *****. She was letting you down nicely man. Quit now.

It means that they are flattered that someone likes them but they don't like you back :(

Its a nicer way of saying no

It means they don't like you.

They don't have time for you anymore. You don't contribute and you're just a drain on their time. You're pretty much useless.

it means they don't fancy you.