Unit will not start any of the wash cycles. Have removed front panel and want to test the AP3178588 fuse. Where is it located?

Here are your parts.
Replacement parts for GU3200XTPS models | AppliancePartsPros.com

See the attachment for the tech sheet. You are missing the last digit on the model number, the attachment is for TPS0 and TPS1.

You said that the unit will not start any of the cycles so I am assuming it is not totally dead.
If it is totally dead, check that the wire nuts (if used) to connect the unit to the house power are OK.
Also that power is getting to the unit.

Turn the breaker off for a couple minutes to see if it will reset.

Check that the door switches close.

If it is an unresponsive keyboard, check the control panel (keypad) using a meter. If it is OK then replace the control board. Still a bit of a crap shoot but your odds are better.

You may want to try the following.
Connector Cleaning / Checking
Unplug the unit
Open it up
Remove the ribbon cable from the keypad to the control board at the control board.
Clean the edge connector.
If it is the type where there is not a connector just the insulation is removed, it can be cleaned with a pencil eraser. Do not use an ink eraser as it is too abrasive.
Check the tracks in the ribbon cable for hairline cracks. A magnifying glass helps here.
Would not hurt to clean the connector on the board. Use electronics contact cleaner or isopropyl alcohol and a tooth brush. Be gentle and do not reconnect till all the solvent evaporates.
Reconnect everything and give it a try.