How do I enter my RMD Nebraska tax withheld

That should be reported on your form 1099-R, in box 14 and/or 17. Just enter the form 1099-R in the federal section of TurboTax under "Retirement Plans and Social Security" and then "IRA, 401(k), Pension Plans (1099-R)."

@swj42    your question isn't perfectly clear....as stated

the  Federally taxable part of the RMD itself may end up in box 14

The NE taxes withheld will be in box 12 of your 1099-R...and should be entered the software with your 1099-R entries.

(Box 15 is for taxes withheld for some local tax authority city/county.  I don't know if NE has any of those, but some states do have local taxes on income)

Yes, you are correct, I was looking at the 2020 form 1099-R, they changed the boxes around!

Ouch...those new box numbers will mess a whole lot of folks up next year...especially if they access this year's answers

...Chaos for awhile.