You're looking in the wrong place. You are looking in Site admin, which handles access to the whole site. You need to be inside a course and looking at the Course admin section. Have a look at this video. (It is from our Learn Moodle MOOC 3.2 version) It starts inside a course, so you need to click onto your course which has the quiz in and take it from there.
General help
Moodle 3.3 - You can not enrol yourself in this course
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