Just add an END at the bottom.

case [Select a Measure]

WHEN 'Count' then

  If attr([Outcome Year]) = 'After Year 1'

  then sum([Attrit After Yr 1])

  ELSEIF attr([Outcome Year]) = 'After Year 2'

  THEN sum([Attrit After Yr 2])

  ELSEif attr([Outcome Year]) = 'After Year 3'

  THEN sum([Attrit After Yr 3])

  END

WHEN 'Percent' then

  If attr([Outcome Year]) = 'After Year 1'

  then STR(ROUND(SUM([Attrit After Yr 1])/TOTAL(SUM([Attrit After Yr 1]))*100,2))

  ELSEIF attr([Outcome Year]) = 'After Year 2'

  then STR(ROUND(SUM([Attrit After Yr 2])/TOTAL(SUM([Attrit After Yr 2]))*100,2))

  ELSEIF attr([Outcome Year]) = 'After Year 3'

  then STR(ROUND(SUM([Attrit After Yr 3])/TOTAL(SUM([Attrit After Yr 3]))*100,2))

   else null

  END

END

You have the correct END for each IF clause.

You need one more END at the end of it all for the CASE statement itself.

CASE [whatever]

WHEN "X" then "something"

WHEN "y" then "Something Else"
END

You need ENDs after both your IF statements and your CASE.

Can you attach a packaged workbook? I fear there are a few different problems here.

Oh.  in the upper half od the calc you are loading SUM(stuff) and in the bottom half its STR(stuff).

It has to be all numeric or all string.

I would take the formatting off the axis numbers.  (Just have 20, 40, 60, etc on the percents and 10000, 20000, etc on the big number.)

And if you make everything string in the calc, you can format the string however you want based on the parameter selection.  Just build the % into the overall string right in the calc.

But if you really need the formatting in the axis, then I would look at making two sheets and swap them based on the value of the parameter.  (It's a common technique.  Two sheets occupy the same container, but only one of them displays at a time.)

You can find video examples of how to sheet swap here:

New series of videos on swapping and popping on a dashboard