51 votes are required to effect the “nuclear option” (if the link below is correct).
Retiring Sens. Jeff Flake (R-AZ), Orrin Hatch (R-UT), and Lamar Alexander (R-TN) all released statements on Friday, opposing the nuclear option to fund the wall (according to the Breitbart article).
So it seems to me that Mitch is saying the votes are not there at this time to bypass the normal 60 votes needed to proceed.
https://en.wikipedia.org/wiki/Nuclear_option