Hi, it's not really a question more to inform or investigate about the reason why.
About this great and helpful article of Wolfgang Beer : Custom percentiles for dashboard charts & Dynatrace API calls.
In the example API call fetches url the name parameter agregationType with only 1 g works for the value percentile. But for other Built-in metrics and aggregations, agregationType parameter with only 1 g don't works and will return the message :
"message": "The aggregation type is not allowed for the given timeseries id."
So be careful to always use aggregationType with 2 g because it always works 🙂
I hope this will help some of you to not loose time, as me 🙂