"Your sentence does not have an object."
Translation:Η πρόταση σου δεν έχει αντικείμενο.
In my lexicon I find for "sentence" "φράση", why is it here always wrong
Because it means 'phrase' or 'expression'.
A phrase is a type of sentence. But not all sentences are phrases.
I'm only a learner - a native speaker might want to correct me - but I don't think φράση and πρόταση are interchangeable.