If you did not substitute for the amount withheld any other monies to complete the Roth conversion of the entire distribution, the amount withheld for taxes is an amount distributed to you and not converted to Roth.
You could potentially complete a late rollover back to the traditional IRA of this amount under Revenue Procedure 2016-47 due to financial-institution error, but it would be up to the financial institution to accept your self-certification that the circumstances would qualify for a waiver: <a rel="nofollow" target="_blank" href="
https://www.irs.gov/pub/irs-drop/rp-16-47.pdf">https://www.irs.gov/pub/irs-drop/rp-16-47.pdf</a>
