I contributed after tax money to IRA to do a backdoor coversion to Roth IRA. The form says "taxable not determined" However TT calculates it as taxable. How to correct it

In order to correct this, you should report that it was a rollover.  

1. Revisit the 1099-R you already entered — you can use the magnifying glass in the upper right of your screen, type "1099-R", and choose the first option "Jump to 1099-R"
2. Choose the page and magnifying glass icon next to the 1099-R  to edit it.
3. Continue until you reach the screen that says, "Did you roll over all of this $xx (Box 1) to another retirement account?".  Answer "Yes"

 

Here are the full instructions to report a Backdoor Roth: How do I enter a backdoor Roth IRA conversion?