I am trying to create a custom advancement that deals with placing a diamond block. I need the player to place a diamond block on any surface, after which the advancement will be complete. For some reason, no matter how much I try minecraft:placed_block doesn't work for me. Is this a glitch? Am I just a idiot? Can anyone please help me out here as I would like to understand what I'm doing wrong!

Hi, thanks for reaching out. Unfortunately this is nothing related to Custom Advancements, but to Minecraft itself. I can only refer you to the minecraft wiki page for the syntax of advancment json files. Hope this helps.