Not a single-group group function in TDengine

Question

I'm writing a sql with partition by in TDengine database.

My statement is:

select app_id, task_name, sum(allocated) as allocated
from (
    select tbname as app_id,
           task_name,
           apercentile(allocated, 90) as allocated
    from yarn
     where ts > now -5m
       and tenant_name = 'xxxx'
 partition by tbname,
              task_name
   ) t
partition by task_name;

It reports an error:

DB error: Not a single-group group function

Why is it working correctly?