One of the inputs to the Formula Node is not wired. LabVIEW does not know what value to use for the parameter. For example, in the following illustration, the Y input parameter is not wired.
One way to correct this error is to wire a data input to the unwired input.
Another solution is to remove the input from the Formula Node by right-clicking the input and selecting Remove from the shortcut menu. If you choose this option, you might have to remove any reference to this parameter from the formula.