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
Food, +1 




all other yields+100%
Defense Bonus for units on tileallows exploitation of any strategic resource on the tile
+10 HP to nearest workable City on construction
adjacency bonuses:
+1
/
to Farms+1
/
to Camps+1
/
to Pastures+1
/
to Mines+1
/
to Quarries+1
/
to Plantations+1
/
to Lumber Mills+1
/
to Logging Camps+1
/
to Fishing BoatsTech Boosts:
+1
, +1
at Iron Working+1
, +1
at Theology+1
, +1
at Economics+1
, +1
at Industrialization+1
, +1
at Flight+1
, +1
at ComputersIncreases WLTKD from Great Merchants, Hurry Production of Great Engineers
Have you tried reinstalling the game? Do you have all the DLC, perhaps it is asking for something you haven't downloaded.