Sometimes there are no photos to go with a memory. It's OK to scrap it - it's your memory which you want to preserve.
Sometimes you aren't allowed to take photos at an occasion, but you may be able to find pictures on-line or in a magazine or newspaper, or brochures, programmes, receipts, or other memorabilia. That's OK too.
I have plans to scrap an unfinished piece of cross-stitch from my childhood, and my first ever wobbly piece of knitting. There will be no photos to go with those, but they are important to me. And I don't know what else to do with them!
They are your scrap books, and your memories. You can put whatever you like in them. Just have fun.