Difference between two dates

Anonymous  are you attempting to do this in a measure or a calculated column?

One way to do it would be with a calculated column, added to the invoice table.  Formula might look something like:

Order Completion Time in Days = 

VAR _Invoice_Date = 'Invoice Table'[Invoice_date]
VAR _Order_Date = LOOKUPVALUE('Order Table'[Order Date],'Order Table'[Order Number],'Invoice Table'[Order Number])
VAR _Result = DATEDIFF(_Order_Date, _Invoice_Date, DAY)

Return
_Result

Anonymous a similar approach could be used with a measure in a table visual, but in order for it to work properly you would need to ensure that only a single "Invoice_Date" and "Order Number" are in "context" (in other words, you need to include Order Number and Invoice Date in your visual). 

I would also use SELECTEDVALUE() to ensure the measure doesn't return anything if there are more than one Order Number and Invoice Date in context.

Order Completion Time in Days (measure) =
VAR _Invoice_Date =
    SELECTEDVALUE ( 'Invoice Table'[Invoice_date] )
VAR _Order_Date =
    LOOKUPVALUE (
        'Order Table'[Order Date],
        'Order Table'[Order Number], SELECTEDVALUE ( 'Invoice Table'[Order Number] )
    )
VAR _Result =
    DATEDIFF ( _Order_Date, _Invoice_Date, DAY )
RETURN
    _Result