I'm not sure if you recently added this or not, but there is currently an incompatibility issue with TerraFarm that throws the same error you are getting. The issue is in the way they register their dirt fillType. It is registered very late in the load process, after UnitConvert has initialized and loaded everything. There is a work around, by registering that fillType the FS22 xml way in TerraFarm, UnitConvert, your map, or any mod really.
Try disabling that mod and see if it still stops working. If it does, I'll have to look into other conflicts. If you need help with the work around, let me know. The next version of UC should resolve that issue.
Update: I just looked at the TerraFarm version you are using and that version apparently does not late register any fillTypes like the github version does. The UC error you are getting still does stem from a fillType being missed by the UC system, however. I think the first step here would be to tell me the most recent mod(s) you've either added or updated, primarily ones that add new fillTypes.
The error is being thrown by the production menu, so it would be a fillType used in a production placeable. I'll have to make a note to add a message telling me the offending fillType if that happens in the next version I'm working on.