form asking for cost basis on 1099B but the box is empty. any reason why this box is empty when that info is needed?

The financial institution that issued you the 1099-B, does not know the cost basis.   If you don't know or can't find the cost basis, the proceeds will all be capital gains.   There is an option in TurboTax (uncommon situations) where you can adjust/correct the reported basis. 

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