Please try:
=ArrayFormula(sumif(mod((row(D209:D)-row(D209)+1),9),0,D209:D))+D209
The intent of +1
and +D209
is to count, but not double count, the value in that cell†. There may be other (and better!) ways to achieve the same result but I aimed to minimise the difference between the formula above and the OP's attempt (except I moved ArrayFormula
to the outside as that may be better practice in general). Minimising differences can help OPs to understand what it was they got wrong (though I admit the reason here was me being lazy, as usual).
† Off-by-one error may be of interest. Whereas the indices of "every 9th" would mostly be multiples of 9
the first entry would be index 1
(1-based), as I understand the requirement.