Idk if this will work for everyone but when I was replacing all my wall with reinforced wall, I couldn’t place blueprint in some spots while other work fine. By accident I managed to place it despite it showing red when I place the blueprint next to it. This work by pointing where 2 wall meet between and it will go back and forth repeatedly between them. Then I press confirm repeatedly until the blueprint have been placed. I can confirm this work for floor as well.