Why didn't Turbo Tax reduce the amount of my refund by $600.00 since I had already received my stimulus?

The recovery rebate is on Line 30.   TurboTax calculates the amount on Line 30 by using the income level and dependents on your 2020 tax return.   Then you are asked about the payments you already received.   The amount you already received is subtracted from the amount you qualify for.   In most cases, the amount of the recovery rebate should be zero.   But if you indicated that you received less than what you qualify for, TurboTax will put that amount on Line 30.   

How do I claim the Recovery Rebate Credit in TurboTax?

Did you enter your stimulus payments correctly in the recovery rebate credit section when you prepared your tax return?   You were supposed to enter the payments you received there so that the software could calculate whether you should receive any additional payments.   If you did not enter the payments correctly then the  IRS cross checks and reduces your refund.