Case Statements in Expressions not working

I am trying to use a case statement in an expression (say and variable) and get the following error as soon as I select case:

I am trying to read the incoming user text and if it contains a word let’s say Paternity or Maternity and it will set a variable that will be used later in the process to preset a field in a service request (trying to pre-determine the type of holiday the user is requesting ).