This message warns you that although you have extended the "Cloze the quiz" time to 15:15 for the student, the quiz will in fact be inaccessible for him after 15:00 due to the Restrictions in the quiz settings. The only way for the student to access the quiz after 15:00 is to remove the "until" date restriction in the quiz settings or change it to "until" 15:15. (Note that there seems to be a bug that if a "from" date restriction in the quiz settings is not specified, the warning message in the override is not displayed.)