IRS adjusted my refund by $600. I filled Turbotax Premier out correctly. IRS says they believe a miscalculation on Form 1040 SR. Any idea why this happened???

If the amount by which they adjusted your return is exactly $600, it very well may have something to do with the second round of stimulus payments that were sent out.  If you receive the $600, but did not indicate it on your tax return, you refund (according to your tax return) would have been overstated by $600.  You should receive a letter for explanation from the IRS with a few days.

You must have had 600 on 1040 line 30.  That would have been for the second Stimulus payment if you didn't already get it.  Did you get both the first and second rounds?  Maybe the second one came after you filed?  The IRS knows they already sent it to you.  But you tried to claim it again on your tax return.