Looks like the credit button aka start button is located on the switch matrix at:
Column 1/green-brown wire/2J2 pin 9 on driver board
Row 3/white-orange wire/2J3 pin 7 on driver board
First, make sure you're getting continuity from the white-orange wire at the credit button (aka start button) switch up to the white-orange wire terminating at 2J3 pin 7 at the driver board. If no continuity, try checking continuity between 2J3 pin 7 and the white-orange wire at any other Row 3 switch (listed below). If you still don't get continuity, there may be a bad connection between the backbox and playfield wiring.
Then check continuity from the green-brown wire at the credit button switch up to the green-brown wire terminating at 2J2 pin 9 at the driver board. If you aren't getting continuity between those 2, try 2J2 pin 9 to the green-brown wire on any other column 1 switch (listed below). If you still don't get continuity, there may be a bad connection between the backbox and playfield wiring.
Credit/start button switch wiring: White-orange wire (Row 3) attaches to the solder lug with unbanded side of diode. The banded side of diode goes to the solder lug for one of the switch leaves. Green-brown wire (Column 1) goes on solder lug for other switch leaf. Compare with other known-working switches as they'll all be wired similarly.
If the wiring looks correct on the credit button, i would check the wiring and diode on the rest of the switches on row 3 *and* column 1 to make sure everything connected properly and firmly (you're looking for a broken off wire or diode leg). I'm suggesting that because a broken wire/diode on any switch can cause other switches on the same row or column to stop working as well.
Row 3/white-orange (found on *both* diagrams I'm posting) includes: "D" rollover switch, "O" drop target switch, "B" rollover switch, "E" rollover switch, and "R" drop target switch (part of the GAR drop targets). You can see them all on Row 3 on the diagram below. (Switches 11, 19, 27, 35, and 43). As suggested above, you can check continuity between the white-orange wire on all the row 3 switches and also from any of them up to 2J3 pin 7 on the driver board.
Column 1/green-brown (only listed on the above chart) includes: plumb bob tilt, the ball-roll tilt, credit button, right coin chute, center coin chute, left coin chute, slam tilt, and high score reset (near diagnostic switches). Check continuity for the green-brown wire between the column 1 switches and also up to 2J2 pin 9.
The schematic is a little funky since all the switches are not listed on the main switch matrix diagram posted directly above. The relevant part of the switch matrix in this case is also located on the first diagram i posted further up (specifically, Column 1).
Either way, just remember *any* broken wire or broken-off diode leg can sometimes cause *all* the switches in the corresponding row or column not to register. A bad header pin or connector at the driver board 2J2 or 2J3 can also cause problems like this. A gentle tug on the wiring and diode legs should be sufficient to reveal anything broken or loose. Hopefully you'll find it's just a broken wire/diode or driver board connector, but it may take a bit of digging before you find the culprit. Also make sure the diode on each switch you check isn't contacting anything else nearby.