I had experienced this little thing long ago, it seems to appear when you try to build far enough from the map’s origin spawnpoint (the platform for example in Smooth Dirt)
hm but i havent moved something far away from the spawn or at least i cant find something on the item finder. a friend of mine tried to place a canvas wall which disappered before it was placed
Items that dissapear like that often land up a massive distance from where you expect. I think this might be a bug as I had it when building a mini golf map. The only solution is to search for them.
Hold down the inventory key (default q) and then click on the item finder on the far right. Now search for an item with a very large distance and this is probably that missing canvas.