Turbo Tax and Credit Karma are coming up with two different refund amounts for my 2020 tax return

Then you have to compare each line and find the differences.

Oh do you mean the refund you received on a Card?  How much is missing?  Did you have the Turbo Tax fees deducted from your refund?  There is an Extra $40 Refund Processing Service charge (45 in California) for that.  ,

Or if the IRS Adjusted your return they will mail you a letter explaining it.  Could it be for a state refund? Federal and state refunds come separately.