I was writing the following sentence and I'm puzzled which one would be the correct sentence. In my humble opinion, the second sentence looks wrong as a comma is not required. The second part of the sentence "I received..," is an independent sentence, and it is better to use a conjunction like which, or that, or maybe a comma is not required at all.
I am writing this letter to express my dissatisfaction on the laptop repair service I received from you service center.
I am writing this letter to express my dissatisfaction on the laptop repair service, I received from you service center.
I am writing this letter to express my dissatisfaction on the laptop repair service that I received from you service center.