Most likely an issue around the wiring of the ball trough.

Lift the playfield and locate the ball trough (where the balls sit).

Check the wiring and plugs that connect to this mechanism.

Very likely a plug has come adrift or a wire has broken off in this area.

If you put it in diagnostics then switch test, if you press the shooter lane switch with your finger while in switch test, does it register? If not, you have a column out in your switch matrix. Column 2 is 2 ball trough optos and 3 ball trough switches and the shooter lane.

If you go to the menu using the buttons inside the coin door, then Select the DIAG (Diagnostics), then SW (switch), then TEST.

It puts up a grid of dots. The ones with squares around them are activated. Try hitting various targets or ramp switches and see them "activate" on that grid. If you press the shooter lane switch (the wire where the ball rests when it comes out initially to plunge onto the playfield), it should also trigger a square on that grid. If you get nothing for the shooter lane, but other switches work, your column 2 on the switch matrix is likely out.

You need to replace the opto board. This is a common issue. Happened on mine, although a different error. Part is like $15