This is entirely a question about JSTL and so it should have been put in the JSP forum. But it's easy to answer, the message says that the "variable" attribute isn't valid in the "c:set" element and that's true. It should be "var" and not "variable".
Also, c:set shouldn't appear in that location inside c:choose. It should be outside the c:choose element. And the code isn't going to work as you expected because you declared a variable named "activationDt" and then tried to use variables named "activatioDate" and "activationDate".
subject: Attribute variable invalid for tag set according to TLD