I just experienced this on my document. Using Adobe Illustrator 20204 v.28.7. Was applying a stroke on a shape and got the message "Can't finish previewing. Could not complete the requested operation." and the document turned into outlines. It fixed itself when I removed the stroke from that one shape and selected "Preview" in the "View". Not sure what caused it.
I just experienced this on my document. Using Adobe Illustrator 20204 v.28.7. Was applying a stroke on a shape and got the message "Can't finish previewing. Could not complete the requested operation." and the document turned into outlines. It fixed itself when I removed the stroke from that one shape and selected "Preview" in the "View". Not sure what caused it.