Hi. Looking at the math, that looks correct to me, but maybe you are misinterpreting what the screen is saying. It shows you have the correct balance (100.00 - 81.58 = 18.42), but that you are spending it too fast and will run out before the budget period ends. Since this is a weekly envelope, Goodbudget expects that on average, you will spend approximately 14.29 per day (slight variation in decimal precision). The little vertical black bar shown above the "for 2 days" in your screen shot is where you would be if you had spent exactly 14.29 per day (by my math, that is at the 42.86 mark). Since the green bar is below that, it shows you have spent money from this envelope faster than anticipated. The statement that reads "You're behind by 24.44..." is just letting you know that if you keep spending at the current rate, you will run out of money before the budget period (1 week) ends. (Note that 42.86 - 18.42 = 24.44...the amount of your overspending). It is not saying 24.44 is your balance or that you are in the hole by 24.44.
This is just a handy tool for you to monitor your spending rate. It is useful for those envelopes in which you have frequent transactions, but not useful for envelopes that only have one or two transactions (you can just ignore it for those).