Oracle Message PLS-00363 - expression string cannot be used as an assignment target

ERRORexpression string cannot be used as an assignment target
CAUSEA literal, constant, IN parameter, loop counter, or function call was mistakenly used as the target of an assignment. For example, the following statement is illegal because the assignment target, 30, is a literal: SELECT deptno INTO 30 FROM dept WHERE ... -- illegal
ACTIONCorrect the statement by using a valid assignment target.