After you replace the batteries like everyone says you will still probably get the error.
First, make sure they are not so old they did not leak. If they did you may have larger issues.
Assuming you got to them in time and put in fresh ones the first thing you want to do is use the button in the coin door to get it into service mode. You will want to go through the settings and set Free Play to Yes.
For specifics either look it up by searching, download the manual or write back. But I believe you need to go into the test mode and exit again for the adjustment error to go away when you turn on the game. Most people do not get this because they change the batteries with the game on to save the settings and high scores.
Here is a link to the manual - http://mirror2.ipdb.org/files/1176/Williams_1986_High_Speed_Instruction_Manual.pdf