It´s ok!

As Germany you dont have any Rubber producing provinces.
So you cant produce rubber. What you do is transform Oil to rubber.
And if you dont have enough Oil (which I guess you dont as Germany) Coal will be tranformed to Oil.
As Germany you ahve a large Coal production in your provinces. So for you to get all the Oil and Rubber you need. Simply transform the coal to oil then to rubber.
How do you do that you may ask? Well it happens automatically. What you do want to check is that your Coal stockpile dont decrease. But it will do taht anyway. As Germany you need to capture land from your enemies to keep your industry going.
I usually conquer Sweden since they have a large Steel production.
One more important thing. In the research tree, you have an Industry tree. if you look through that you will see you can upgrade your industry to convert Coal to oil and Oil to rubber more effiently.
I think taht from the start you convert 10Coal to 5Oil.
and thoose 5Oil becomes around 2rubber.
Bah, its not as hard as it seems.
Check the ledger and see what your missing and how many converts youre doing.
remember that when youre at war all world market gains go down horribly.
If you say have a trade at 200Coal for 100Rubber before the war then taht will only be 200Coal for 10-15Rubber when youre at war with anyone.
I suggest you cancel ALL your world market trades. Atleast thats what I usually do.
Good luck.