Recovery Rebate Credit

I filed a 1040-SR form though Turbo Tax in March. Did rec. my state refund in April.  Did not rec. my Federal refund until the beginning of August.  It was less than calculated on my return.  On August 23, I received a Notice from the IRS saying that basically they deducted the Recovery Rebate Credit of $600. from my refund

stating a miscalculation.  I had entered 600.00 on line 30.  I do not understand the "penalty".  

I have had one phone conversation with an IRS rep.regarding this situation and did not get an explaination.

Please help me understand what has happened or help me recover the 600.00.