These errors occur when the first argument to plots[pointplot] or plots[pointplot] is not a set of points of numeric values specified in the proper format.  For the correct syntax, see the plots[pointplot] and plots[pointplot3d] help pages.

Example 1

The variable  is undefined:

Solution:

Defining the variable fixes this error.

Example 2

Lowercase is used instead of uppercase

Solution:

Using uppercase corrects the error.

plots[pointplot], plots[pointplot3d]