If it’s your main source of protein.

A 200Lb adult needs a minimum of 140g of protein daily to remain healthy.

A single serving of peanut butter has 190 Calories, and only 7g of protein.

If that 200Lb adult was getting just half of their protein from peanut butter, they would be consuming 1,900 calories in the process. Even if they are active enough to justify that caloric intake, they would still be consuming 160g of fat, which is double the daily recommended amount. It's the nutritional equivalent of drinking a 2/3 cup portion of cooking oil every day.

Tl;Dr: Do not make peanut butter your main source of protein.