why is the word "was" such an issue

You haven’t given us much to go on, Susan, regarding “such an issue.”  I'm not fond of making assumptions, but will assume that it has to do with Grammarly's software pointing out the passive voice. 

Grammarly’s software is used by a lot of students.  Many teachers and instructors require papers to be written in the active voice.  Though it is not grammatically incorrect to write in the passive voice, and sometimes that is the best choice, it is usually more pleasant to read the active voice.  The software catches the passive voice to assist students who might otherwise lose points on their papers. 

This sentence could be changed to:
The study required maternal consent for participation.   

To move it to active voice you would write, "To participate in the study, maternal consent [was/is] required."  Tense of the verb would depend on what you're using in your paragraph or paper.  Whether you participate in the study today or participated last month, consent is required (assumption).

Patty's sentence is a grammatically valid, active version. However, since it was (probably) the people undertaking the study who stipulated that maternal consent was required, not the study, the sense has been changed. You should be careful not to scarifice logic and clarity in the persuit of a "passive-free" text, as in this case you would probably lose marks as a direct result.

Using the passive voice is necessary in this instance. Your sentence is fine.