Should I get a second opinion if the doctor says that there is nothing wrong with my knee when the MRI says that I have a displacement of the root?

Depends. If your meniscal root is actually torn and displaced in an otherwise normal knee, it should be repaired. Question arises whether it is actually torn(radiologic interpretation) or if it can actually be repaired. That should be discussed with an orthopedic surgeon.
Helpful. Second opinions are always helpful - perhaps consult an orthopedic surgeon or another radiologist to review the clinical data + MRI scan.
2nd opinon is fine. As a patient, you should always feel free to get a 2nd opinion - and most docs welcome a 2nd opinion. That being said, just because there's something seen on the MRI doesn't mean it's what's causing your pain or symptoms. We often pick up "incidental findings" on imaging tests that are harmless. A good physical and history is key to knowing if the MRI means something or not.