In the program interview you were asked how much stimulus payment you got in advance and based on that figure the program computed the allowable credit for any missing amount you are eligible for.... but the IRS disagrees with the amount you say you got in advance so they have adjusted the credit on the return. Due to this adjustment you now owe so you should pay it ASAP to keep the penalties/interest from accruing further.
If you believe the IRS is wrong check your account at the IRS to see what the IRS says they sent you ... if you did not get the payments they say were sent then you can have the payment traced.
Go to this IRS website for how to start a trace - https://www.irs.gov/newsroom/questions-and-answers-about-the-first-economic-impact-payment-topic-f-p...
And view your account at the IRS to see what the IRS shows for you: