They are both correct. It depends whether you consider what comes before the verb to be one subject or two. One subject: The phenomenon ( of cyberbullying and its corresponding impact) has.... or Two subjects: (The phenomenon of cyberbullying) and (its corresponding impact) have... I'm not sure what you mean by "parallelism" here, but I hope my explanation makes sense. The "issue" you refer to is probably simpler than you think - it's just a question of deciding whether you've got a singular or plural subject.

I have tested "Grammarly" for use with non-native speakers and I have found it to be unsuitable for all except near-native speakers. I am happy to explain more.

Both are correct, yeah

Cyberbulling and its impact being 2, you'd use "have"

Cyberbulling and its impact being considered one thing all together or according to the last conjugation you'd use "has"..

But i would say "cyberbulling and its impactS have".. so both options would be "have" anyway..

Xx

Has is correct or at least sounds natural and instinctive to native speakers. A native speaker would use have for a third person he/she/they have. "you have sparked a heated debate about, the the phenomenon of cyberbullying and its corresponding impact" https://www.ecenglish.com/learnenglish/using-have-and-has/ PLEASE NOTE I am not vouching for this English language school nor have i checked the answers on the test. There are often errors on the Internet, everybody makes silly mistakes. Even teachers at reputable Universities. I posted the link because it is relevant to this question.