Compare dates

If you use a date column from the same table in your slicer, it filters that table and prevents your desired functionality.  You can make a simple DAX table to be used in your slicer instead, then store the selectedvalue in a variable for comparison in the rest of your expression.

SlicerDates = DISTINCT('NPLM Import'[Plan R@R ...])

Ø Annual Turnover (EUR) Done = 
var thisdate = SELECTEDVALUE(SlicerDates[Plan R@R ...])
Return 
IF(
   MAX('NPLM Import'[Plan R@R (internal) Date]) >= thisdate, 
      IFERROR(SUM('NPLM Import'[Yearly average turnover (EUR)]), "")
)

If this works for you, please mark it as the solution.  Kudos are appreciated too.  Please let me know if not.

Regards,

Pat