Question: Error in optimization? what parameter should ii change to get graph concave wrt tau2?

I'm encountering an error in the optimization process when adding constraints C1 and C2. I've attached the sheet for reference. 

Q4.mw
To obtain a graph similar to the one below, which parameters in the data should I adjust? As you can see in the attached sheet, the graph is concave with respect to tau1 but not tau2. What changes should I make to ensure the graph is also concave with respect to tau2?

Dummy graph:-

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