You can use an expression of the form .property=some_value only within a WITH ... ENDWITH construct.
This error might also occur if you are using .NULL. as an expression.
Put quotes around the .NULL. to keep it from being identified as a property.
You might have forgotten an ending period with a value such as true (.T.), false (.F.) or a null value (.NULL.)
Check for matching periods on either side of the constant.