Put a real terrain behind the plan

Draw on the actual terrain instead of a blank sheet.

Admin Web — Terrains, or the map editor’s Terrain and OCAP menus

How to do it

Best: upload it. Go to Terrains (linked from the Maps page) and upload the same tile archive you prepare for OCAP — a zip or 7z with the numbered folders 0/ 1/ 2/ … in it.
Then open a map and use the 🗻 Terrain menu in its bar to pick it.
Or point at a running OCAP server: in the editor’s 🛰️ OCAP menu, press List the terrains, pick one, and import it.
Either way the terrain’s Arma coordinates come along, which is what makes the export land in the right place.

Prefer the upload for anything shared outside the unit. A share link is for people who do not sign in here, and expecting them to reach your OCAP instance as well is how a link ends up showing an empty sheet.
If the archive has no map.json, you are asked for the world size in metres. Without it the tiles would draw but the Arma export would be nonsense.
Levels deeper than a planning sheet needs are dropped, and the reply says how many — the terrain is meant to be smaller than the file.
A terrain cannot be deleted while a map is drawn on it, and the refusal names the maps.