This is often due to players not having a location to spawn.
Option 1, if you want to temporarily test a map giving InsideTerrain Violation on player spawn, terrain_protection 0 can be set at console or in server.cfg. Not recommended to have this enabled on a server with players.
Option 2, if the map is intended for players and not just for testing, use the RustEdit.io map editor. With the Terrain Painter, use the Conditional Brush, Enable Heigh 0 - 1, and paint Tier 0 over the beach area that players should spawn.