when it says mic they are referring to the big connector(Most mics use them) you are using the line connection so pushh it in and see what happens. If the problem persists try a different mic and or cable then if that does not work it is the mixer

I suspect the RCA socket or jack socket is bad, I have occasionally had these fail on other type of equipment, I would see about having new ones installed.

Press the mic / line button. It was still up 🤔

The op 2068 is damaged on that channel.

Just click on line, because you re using jack microphone 🤷🏻‍♂️