Switch to control mode is greyed out

Decided to start a new discussion hoping someone can help.

Hi.

I have a problem with screen sharing where the option to "switch to control mode" is greyed out. I can only observe but not control the mac. I have 4 macs at home with the same set up "it looks like the same set up" and it only happens on my wife's new iMac.

I appreciate your help.

Yvon.