One of the commonest errors at any level of writing is to leave out a necessary bit of a sentence. Sometimes writers miss out an important idea because it is so obvious to them - but the reader has no idea what is being said. (Sometimes, this can be a word like 'not', whose absence changes the meaning of a sentence totally!) Good proofreading should catch these errors.
It is less obvious sometimes to students - though it often attracts the Tutor's attention - when one of the essential building blocks of grammar is left out. The most essential of all are the subject and the verb.
The verb is - you may have been taught when younger - a 'doing word'. More accurately, it is a word that can change its ending to match its tense (e.g. he does and it is are the present tense; he did and it was are the past tense), or, in the present tense, its number (whether it is singular or plural: does she, or do they?)
The subject of a Sentence, in Grammar, is not the same as the logical topic. Grammatically speaking, the Subject of a sentence is the 'person' who is performing the action of the verb - who is 'doing' the [doing word]. In the following examples, the Subjects are in bold: Wellington won the battle of Waterloo; Scotland won the World Cup (I wish!); It is raining; There is something for you. (The last two are examples of dummy subjects: although they are essential grammatically, they do not actually mean anything.)
Nake sure that you have a (grammatical) Subject and a verb in every sentence that you write in academic English.