You can’t be in two places at once.
This idiom is also translated on tatoeba as ‘you can’t have your cake and eat it too’, which I think is a bit better (but since I’m not a native speaker . . . ) At any rate, on a Greek language forum, one person said it meant “Ότι κάποιος δεν μπορεί να τα έχει όλα όσα ζητά”, i.e., that ‘someone can not have all the things that they want’, ‘you can’t have everything’.