Insufficient resources

I have the Impreza from cat C. Upgrade to lvl 4 on the engine costs 9.5k. Three times I have passed that and upgraded other parts after getting this message. Now I have 12k and it still says ‘insufficient resources’. What the hell is that? A bug?
Comments
I have the same problem
What have you done? Can not find the answer on your question. Thanks.
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