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You also need to set the gametype in the world properties (menu bar: View >World Properties > Game Type) for both PIE (play in editor) and supported/default game types.
As a general note, avoid using Terrain for your maps, as it is a legacy item. Use Landscape.
https://udn.epicgames.com/Three/Landscape.html
4) Saving and cooking a map:
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Once you've tested your cooked maps on a server, you're ready to upload them. If a map is uploaded to the Workshop, people will be able to find, rate, and comment on it. Perhaps more importantly, server operators will be able to download the map from Steam, and any users who connect to servers with the map will automatically download it from the Workshop. This is pretty important.
See Uploading a Map or Mod to the Steam Workshop for information on how to upload files to the workshop.
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