I'm trying to fulfill the steps beneath, but I see no CMYK values in the screenshot beneath?
Search for Convert All Spots to CMYK and run Fix. Now all colors in your document are CMYK.
Scroll to the page 22, as this is the first page with background fill you want to get rid of. Open again Tools → Output Preview and read CMYK value of the background. As I was converting Pantone with simulation to U.S. Web Coated (SWOP) v2, my CMYK values for background are 10:2:0:5. It's ok, if you will have different values. Just write them down as you will need to use them later.
What have I done wrongly? I screenshot the steps beneath.