I have a1099-R converted to ROTH. TT desktop's Form 8606 shows I owe tax. Previous years TT 8606 has been correct (Line 15c = 0), what is different this year?

If you have made non-deductible contributions to the Traditional IRA and then converted it to a Roth IRA, be sure that your Form 8606 either shows a previous basis in your Traditional IRA (line 2) or a non-deductible contribution made during 2023 (line 1).  Having non-deductible basis in the Traditional IRA before converting it to a Roth will make it non-taxable as long as you do not also have pre-tax (or deductible) contributions in the Traditional IRA.  This is commonly called a Back Door Roth IRA Conversion.  

Take a look at the following TurboTax help article and be sure you have followed these steps:

How do I enter a backdoor Roth IRA conversion?
 

If you were simply converting a Traditional IRA without a cost basis to a Roth IRA, then the conversion is taxable.