Grammarly will identify an issue with subject-verb agreement is the subject of a sentence and its verb do not agree in number. Singular subjects must be accompanied by singular verbs and plural subjects with plural verbs. Below, Grammarly has identified a possible subject-verb disagreement:

The student has written the following:

The percentage of students who called in sick and the number of students who were marked as truant was proportional to the students’ degree of seniority.

Since the subject of the sentence has two parts (percentage of students and number of students), it is a plural subject. The singular verb, is, does not agree with the plural subject. The student can correct the sentence easily by using a plural verb:

The percentage of students who called in sick and the number of students who were marked as truant were proportional to the students’ degree of seniority.

The Writing Center has an excellent overview of subject-verb agreement at Subject-Verb Agreement.