That one doesn't seem to have an enrolment key - it is the enrolment key that you should click into and remove but it looks as if there is no enrolment key. Can you post a screenshot of where you see the key icon and which course it is? (Also, you have Allow existing enrolments set to No there, which means people cannot access the course- I presume that's intentional?
General help
getting rid of the key symbol
A vitát lezárták, már nem szólhat hozzá.