This error occurs when the unknown of the differential equation (indeterminate function) is not indicated and more than one function of one variable is being differentiated.

By default, if only one function is being differentiated and the indeterminate function was not specified, the differentiated unknown is used as the indeterminate function.  However, if more than one such function is present, you must indicate the unknown of the problem.

Example 1

In this example, the indeterminate function could be either $y\left(x\right)$or $z\left(x\right)$; however, no indeterminate function has been specified.

Solution:

Specifying the indeterminate function, in this case, $y\left(x\right)\,$corrects the error.

Example 2

In this example, the indeterminate function $g\left(y\right)$is incorrectly specified as $g$.

Solution:

Specifying the indeterminate function as $g\left(y\right)$corrects this error.