Date diff function is not recognising the date columns from the table

If this is a measure, you need to give it some type of context. Right now, you are reffering to a whole column, so it does not know which value in the column to use.

If you plan on using this in a table or graph, using the function "Selectedvalue" will add the current context.

Try this:

DURATION = 
   DATEDIFF(
           SELECTEDVALUE(sai_duedate) ,
           SELECTEDVALUE(sai_startdate) ,
           day
   )