The cursor is not on a program line that contains a valid
expression. The Instant Watch command expects the
cursor to be on an expression whose value you want to see.
 
Choose <OK> on the error message dialog box and reposition
the cursor within the expression you want to watch.
 
Tip: In a complex expression, it may be hard to decide
     where to put the cursor to watch a particular
     variable or subexpression. You can unambiguously
     indicate what the Instant Watch command should watch
     by highlighting it. Just place the cursor on the
     extreme left or right end of the expression and
     pressing SHIFT+RIGHT or SHIFT+LEFT.