Are you trying to build them with a worker? because they can only be built by expending a GG/GM/GS/GE. Also you can build them anywhere and they grant additional yields to adjacent improvements.I've come back after a year off. Updated to VP 3.10 and updated this mod to v87. I tried one game with the celts as I had never seen oppidums. I got the research and couldn't build oppidum. I couldn't figure out why, but after a bit gave up that game. But was I missing something? You should just be able to build them on strategic resources correct? Is there a prerequisite tech? The help file doesn't say there is.
Spoiler :
UI - Oppidum (Great Person Improvement):
Can be built by Great Merchant, Great Engineer, Great Scientist, and Great General. Does NOT replace other GPTIs
+2







+100%

allows exploitation of any strategic resource on the tile
+10 HP to nearest workable City on construction
adjacency bonuses:
+1


+1


+1


+1


+1


+1


+1


+1


+1


Tech Boosts:
+1


+1


+1


+1


+1


+1


Increases WLTKD from Great Merchants, Hurry Production of Great Engineers