Deductions & credits

I received an IRS letter changing my 2020 return; specifically for the Recovery Rebate Credit. I used the TurboTax form for computation and it appeared correct. We were over the $150k income, but the computation showed a calculation based on 5% of the overage.  The IRS letter shows 0.00 for other credits from 1040SR. But, the TT form shows a credit. To make matters worse, the letter gives us to 9/27 to make payment, but we only received the letter on 9/20. I've called the IRS number several times and get the same response - "given our high call volume we can't address your issue; call back on the next business day". I'm running out of days. I would appreciate any assistance.