Changing 3 way switch to a half 3-way switch half outlet, missing a neutral
Most three-way switch circuits from homes of the 20th century will not have a neutral at the switch box.

The white wire should be reidentified as blue or another color. It is either the hot feed to the 3-way circuit or the switched leg to the lights.

The only way to have a receptacle at this location is to add another 2 wire cable from a source having an actual neutral.

Good luck!