As I understand it, there are some toxicity concerns related to anti-pd1/pd1 drugs and elevated liver levels. However, they are somewhat rare and typically controllable if noted. Thus the reason they give you a blood test every time you come in. My understanding, and you should simply consult your doc to get the in-person, educated and specific-to-you answer, is based on these elevated levels.
However, the easiest way to deal with fatty liver, if, as is typical, it is caused by weight, is through diet and exercise and to the extent you can do that, you certainly won't hurt your treatment. Similarly, you should consult with your doc regarding weight loss and/or reducing/healing your fatty liver if that is the primary cause. If it's caused by something else (alcohol or liver diseases, you should similarly discuss this with your doctor, although I hope that they already know).
Good luck!
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