I had a similar problem on this app. I typed in a goal that was a bit extreme..to lose 30lbs in 3 months and it told me my calorie allowance was -1000! It started telling me I'd gone over my limit when I entered my morning apple! I finally had to uninstall the app then reinstall it to change my goal. It really won't let me lose more than 1.5 -2 lbs/week. Try reinstalling the app.
Your calorie allowance is based on your weight, height, age and activity level. The heavier you are, the more your calorie allowance will be, because your body needs more calories to maintain weight.
Once you've calculated your BMR (Basic Metabolic Rate), which tells you how many calories you need to maintain your current weight, you have to subtract calories in order to lose weight. For instance if you want to lose a pound a week, you would subtract 500 calories/day from your budget, making it 3500 instead of 4000 calories/day. If you want to lose 2 pounds/week you'd have to subtract 1000 calories/day from the 4000 calorie budget.
But first you have to make sure your calculated budget is accurate. I can help you do that, if you'd like to provide your current height, weight and age; you've already said you're sedentary.