Add IF not zero to calculated measure (while dividing)

That is because you are having a scenario where the sum of GA[Clicks] is zero and the measure is attempting to divide by 0.

How you want to handle this depends on your report.

Usually you want to have your data setup so that you do not get div by zero.

Rev per click =
VAR CostSum =
    SUM ( GA[Cost] )
VAR ClickSum =
    SUM ( GA[Clicks] )
RETURN
    IF (
        ClickSum = 0
            || ISBLANK ( ClickSum ),
        ERROR ( "Div by 0" ),
        IF ( CostSum > 0, CostSum / ClickSum, BLANK () )
    )

However if div by zero is a normal case then you can use the divide function to replace the errors by blanks

Rev per click =
VAR CostSum =
 SUM ( GA[Cost] )
RETURN
 IF ( CostSum > 0, DIVIDE ( CostSum, SUM ( GA[Clicks] ), BLANK () ), BLANK () )