because you created the part from the beginning with surface tools, you need to use the hyprid modeling technique like extrude up to surface
Looks like you used the surface tools to create the part and the parting surfaces.
If you wanted to proceed with the surface tools and continue making splits, it's more features and more work.
The better option is to create the part in solid, then use the mold tools to create the split (It's easier and faster as 1.2..3..)
Just make sure you delete Surface-Trim5, or else you won't be able to form a solid. It's a little leftover sliver face somewhere that doesn't seem to be serving any purpose.
I've managed to figure it out without making it solid, turns out the solution was far easier than what I was doing and I didn't need parting lines or any of the mold tools really.
I simply made two rectangle sketches in front and behind the wheel at the depth I wanted the final die to be and "solid extruded" it up to the front plane (where I want it split). Then I used the "Cut With Surface" tool and cut each of the two halves of the steering wheel into the solid extruded blocks to become the dies and got the desired outcome.
Then I put each die into it's own part and away I go.