Value between Two Dates

If you want the calculated column in sales data, you have to work in this part of your DAX:

VAR _start = 'Finance Data'[Sales Letter A Date]
VAR _end = 'Finance Data'[Sales Letter B Date]
VAR _key = 'Finance Data'[KEY]

This is bringing the entire column, not a single value, thats way you got the error: A single value for column 'Sales Letter A Date' in table 'Finance Data' cannot be determined. This can happen when a measure formula refers to a column that contains many values without specifying an aggregation such as min, max, count...

if you use aggregations suc MAX, MIN in that part of your code as shown bellow, you will not get the error:

VAR _start = MIN('Finance Data'[Sales Letter A Date])
VAR _end = MIN('Finance Data'[Sales Letter B Date])
VAR _key = MIN('Finance Data'[KEY])

I used MIN function, just for example, reform as you need.