I’ll bite on this one!
I’m not as good as Sam or Joseph on Overtone but I think the issue here is that you can’t use rotation in the synthdef like that. That error is saying buf-wr:ar is expecting a UGen, not a Clojure array. If you need to do maths inside the synthdef like that I’m sure there are ways but… I was going to suggest hard coding the initial channel to get started? Give that a shot and let me know if you have any other questions.