You can do a Traditional IRA for 2018 tax year, and pay the IRA by 4-15-2019.
You would have to do an Amended return to do this.
You cannot take a 401K paid in 2019, on the 2018 return.
Once the return has been filed it cannot be stopped (unless you get extremely lucky and IRS would reject it for some reason).
So you have to wait for the return to be processed and you get your refund.
Then approx February 28th you can sign back into TurboTax and do the Amended return.