It shows up if you are writing a sentence without the main components : subject, predicate and object. It will also show up if a sentence is too wordy with lots of parenthetical expressions without the proper use of commas, semicolons, colons , etc. So it detects your mistakes and asks you to revise it .

This means that your sentence is not complete. For example, I could say "If you are here." This is not a complete sentence because of the word "if". There is no conclusion to the "if" part.

Significa que você não escreveu uma frase completa. Então você precisa de usar um sujeito e um verbo pelo menos.