Access doesn't provide much help when you type an expression into a Field cell or Criteria cell. It doesn't provide function syntax; it doesn't match parentheses (as, say, Excel does when entering formulas); it doesn't give you an easy way to pick out field names; and its error messages can be distressingly vague.
There isn't much that can be done about the vague error messages, but Access does come with a tool that can ease some of the burden of expression building. The tool is called, appropriately, Expression Builder and it works according to the following steps: