Hi!

Why can't I place buildings or resources anywhere, they're just red? I have starting locations.

This problem is only in new maps, opened pre-made maps works fine, so what am I missing?

Assuming you are using Scmdraft2, in your map, click the Options menu - Units - untick "Require Mineral Distance".

I was using the "installation" terrain theme. Apparently you can't build on it in-game either, and the creep doesn't spread.

There is actually 1 buildable tile for installation, I think if you look in the wiki or in these forum threads you'll find it, assuming someone doesn't come here and give you the answer directly.

http://www.staredit.net/wiki/index.php?title=Null_Terrain

I'm assuming you found this, but I'm posting anyway since it's not explicitly stated. You can preplace buildings anywhere and stack them on any tileset, but units will give an error if actually unplaceable when the map is started. In SCMDraft, go to Options > units > place buildings anywhere, as well as a number of other placement options that are useful.