❓ Why can't players place or break blocks in my arena?

#Answer

Building is only allowed when the arena will regenerate, and only inside the Battle Region. If either is missing, the block is cancelled.

Enable regeneration on the arena, and make sure the Battle Region actually covers the area players build in. If regeneration is already enabled, disable and enable it again so the schematic is rebuilt.

Also check that no arena rule, WorldGuard flag, or spawn-protection is blocking it. Spectators can never build.