Yes, you are correct in that the minimum weight is 80 pounds. The Healthy Body Calculator® (HBC) includes safeguards for people with low body weight in particular eating disorders. Yes, you could put in 80 pounds as that would still give you an idea of a bodyweight goal of around 100 pounds at 5 feet tall. Select gain 1 or 2 pounds per week and I would recommend 1 pound.
If you want to gain weight, click on HELP Healthy Eating for Life Plan on your results page of HBC. Your data will be copied from HBC into HELP where you can select the type of milk, meat, beans or not, snacks or not. HELP will create a plan for you to follow to gain weight. I would encourage you to eat snacks. Choose a wide variety of meat or beans, dairy, whole grains, fruit, vegetables and healthy fats like olive and peanut oils.
Since you probably don’t want to gain just fat, I would encourage you to talk to your doctor about exercise s/he would recommend for SLE (systemic lupus erythematosus). SLE is an autoimmune disease where your body attacks the collagen (connective tissue) all over your body. Many people with SLE are on prednisone (an anti-inflammatory drug) which can cause an increase in blood sugar and/or blood pressure as well as muscle loss. Ask your doctor to see a physical therapist who can recommend specific exercises for you that will help maintain your muscle mass as well as gaining muscle rather than fat as you gain weight. If you are on prednisone, I would not recommend eating sweets or salty snack foods.
If you have not talked to a dietitian, please ask your doctor to see a dietitian. S/he can read your medical chart which I cannot and make more specific recommendations.