With any sort of error you should copy from the log the entire code of the procedure or data step that creates the error along with all notes, warnings, messages and errors. Open a text box on the forum using the </> above the message window and paste the copied text.
The text box is important because the message windows will reformat text meaning what we would see here might not actually be what was in the log.
Most of the time that and similar errors means you either did not spell the variable name correctly or used a different data set than you meant to that does not actually contain the variable. Less likely but still common is an incomplete statement above the place that error is reported that makes SAS think some other word is supposed to be a variable name.