Syntax Error When Using Divide Function

I'm trying to create a new measure that calculates the estimated number of cases deliveried using the DIVIDE function, however I continual receive the same error message regardless the syntax I use. 

Est.Cases = DIVIDE('Actual Sales'[Qty], 'Product Detail'[Units per Case] )

The error message says that it "can not find the field [Units per Case]". 

I've tried so many different ways I'm not even going to attempt to list them all, I always get the same error message.  

Any help would be appreciated. Thanks.