I'm at level 8. I have a full charcoal storage unit but I keep getting prompted to produce more. Can..

The game likes to prompt you if your Charcoal Factories run out of lumber. But at your level the max upgraded Charcoal warehouse will provide enough. So you can definitely ignore the messages. As you level up you will need to use the factories to keep up with production and not run out of charcoal, although if you do not then you will only suffer a lowered production rate on the other resources. You can still ignore the prompts though.

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