Please tell me if this is a valid sentence

[rach6311]

Lijepa ta neka djevojka spava

[kibo]

Not really. It depends on what you wanted to say.

If you wanted to say "Let that pretty girl sleep", it should be "Neka ta lijepa djevojka spava". In that case "neka" is an imperative particle (similar to the English "let")

If you wanted to say "Some pretty girl sleeps", it should be "Neka lijepa djevojka spava". In that case "neka" is an indefinite adjective/pronoun. But in that case it's not really logical to use a demonstrative as well.

Maybe there's a third possibility, dunno.