If you are getting this message in JMP 8 using scripts that were developed in JMP 7 or earlier, you are probably trying to send a message to an object of a data table which does not have the current focus in JMP.  For instance, I have a script which opens two data tables at once, combines them into one table, deletes them, and then keeps doing this behavior for several more tables. Eventually I build up a couple of open data tables, but then attempt to manipulate a data table that I had opened previously which doesn't have the current focus. In order to work around this somewhat annoying "feature" in JMP, just make the table that you want to manipulate the table with current focus using this JMP function:

Current Data Table(<name of the table or variable representing the table);

or for more concrete examples:

dt = "CustomerTable";
Current Data Table(dt);

or even this style will work:

Current Data Table("CustomerTable");