Change time display - only frames option

Dear all,

First time I ask a question here, but I'm really stuck. I want to change my timecode display format from frames to seconds. I found numerous tutorials, however they don't work for me. If I take a look at my sequence settings, the only option I can choose is ' frames'  and all the other options seem to have disappeared. I added a screenshot. Can anyone help me with this problem?

Thanks! Regards, Inge