Not able to add anything to my slideshow. iMovie giving me a message that says “ you cannot add video clips to existing audio-only compound clips” every time I try to add a photo to a slideshow. It will not let me add anything.

I've never seen that message before until this last week when you and one other poster reported it. See this link:

you are combining audio and video clips o… - Apple Community

I don't know what could be causing it. You might try contacting Apple Support by clicking on the Get Support item in the upper right hand corner of this forum's screen and navigating to a place where you can enter your p;hone number.

-- Rich