A-B Looped greyed out

[pisomojado]

I can't get the AB loop to work. I'm on Mac 12.1 and the video is an MKV file. A to B Loop is greyed out, and the hotkeys don't work either. There is no "Advanced Controls" button that I can find. Repeat One and Repeat All both work fine even though they are also greyed out. Please advise on a detailed guide for this feature that works.