Check your coin switches to make sure they aren't stuck closed... also might want to check whatever it is on the board that controls/checks inputs... might have a line on it stuck high/low and telling the game the coin switches are stuck... bad RAM could cause it, but it's unlikely IMO. But... I've seen stranger, we have a Monkey Ball that has a RAM error and the only thing goofy about the game is that when you turn it on it has 28 Credits, every time... anywho, rambled there.
If you don't have a multimeter to check the switches, just unplug them and the turn the game on. If you still have a "coin error" with the switches unplugged, then you've got a board error.
I remember again -> if there are NO mechs present you have to connect the coin meter pins on the JVS with a 100ohm resistor to 5V, that's it and the "coin error" disappears.