@spIash_damage I used that same walkthrough to get them all, I have even gone through the walkthrough 3 times to make sure I did not miss anything.
Based off of a diagram of the bookshelf that I found the issue is with one of the Lunar Landing Gnomes but I have played the map in multiple ways and using plants and zombies to get the Gnomes with no luck. I spoke to a few people that had the same issue with the exact same one and it seems like there is a glitch on a couple of the Gnomes.
Some people started over and when they did it that time it worked and some gave up, my kiddo is a bit bummed but we will try with another account and see if we experiance the same issue.
