I agree that losing 40 lbs in 2 months isn't a healthy goal, which is why the app isn't giving enough information. Not only will your skin sag and be flabby, if you lose that much weight too quickly, but you won't get the right nutrients and be healthy either.
That isn't a healthy goal. You shouldn't lose more than 10 Ibs per month. Losing too much weight will cause your skin to sag and look flabby, you are better off losing this amount of weight in 4 months .
May i ask what is your height and weight currently?