Missing data in Firstnonblank

I'm not sure exactly, but the code above isn't valid at all. You have no columns. Paste your code in DAX Formatter by SQLBI and copy the results back here.

But the FIRSTNONBLANK() function, what is the ,1 doing? You cannot evaluate 1 and get anything but 1. FIRSTNONBLANK() should be:

FIRSTNONBLANK(Table[Field], [Measure or Expression]), so 

First Date =
FIRSTNONBLANK(
    'Date'[Date],
    CALCULATE(
        SUMX(
            Sales,
            Sales[Price] * Sales[Quantity]
        )
    )
)

Will return the first date in your model for which there are sales.