Oracle Message PLS-00321 - expression string is inappropriate as the left hand side of an assignment statement

ERRORexpression string is inappropriate as the left hand side of an assignment statement
CAUSEThe expression does not designate a variable that can have a value assigned to it. For example, the function SYSDATE cannot appear on the left hand side of an assignment statement such as SYSDATE : = 01-JAN-1990;
ACTIONCorrect the illegal assignment statement.