As with many build situations in Conan Exiles, there is an order in which some pieces need to be placed to work properly. When you want to use the Angle Brace or TriAngle Brace build pieces, you will want to:

1 - Place the wall
2 - Place the ceiling tile(s)
- for a square ceiling just place the square ceiling tile
- for a wedge ceiling place the wedge ceiling tile at the wall, then place the two ceiling tiles connected to it
3 - place the Brace build piece which should snap to the wall and ceiling granting additional stability to the subsequent attached ceiling tiles

TLDR: Build the walls and ceilings first, then the Brace and finish extending your ceiling build.