Rubber is missing from the list of upgradable resources. So, you can’t build sawmill on Rubber as intended.

Easy fix

Go here : Steam/SteamApps/workshop/content/289070/1369684991

Edit File : Res2_Sawmill.sql (notepad++ is good for this)

Find this section : -- Improvement_ValidResources

Add in this line :

UPDATE Improvement_ValidResources SET ImprovementType = 'IMPROVEMENT_RES2_SAWMILL', MustRemoveFeature = 0 WHERE ResourceType = 'RESOURCE_RUBBER' AND ImprovementType = 'IMPROVEMENT_LUMBER_MILL';

Save, reload game. Enjoy