Is it possible I can’t gain any weight cause I may have low testosterone? I am 110 pounds.

It . It may be possible. I would make an appointment to see your primary care physician to discuss this.
Possible . Possible but not likely. Besides low testosterone, infections such as intestinal worms/parasites, TB & hiv/aids, cancer, hyperthyroidism, and other chronic disease conditions can sometimes make it difficult to gain weight, although that isn't usually an issue in this country. Don't forget that nutrition plays a major role in weight gain/loss. We usually estimate that 3, 500 calories (kcal) equals one pound of weight. In other words, energy in has to exceed energy out by 3, 500 calories for their to be a one pound weight gain. Likewise, there also has to be a net 3, 500 calorie loss with energy out exceeding energy in for their to be one pound of weight lost. Finally, while we use body mass index (bmi) to diagnose normal vs overweight vs obese vs morbidly obesity, that calcuation doesn't take into account sex, bone structure, and fat vs muscle. In fact, it's better to be a lean 110# rather than a fat 110#. Regardless, go talk to your family physician about appropriate testing & diagnosis. Good luck!