Question

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

I already paid taxes on it

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

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"
Choose the page and magnifying glass icon next to the 1099-R to edit it.
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?