You are missing, or just not showing GetStatId(), don’t know if that will resolve the error but it is required:
Otherwise it looks fine.
https://orfeasel.com/implementing-mu…eading-in-ue4/
FORCEINLINE TStatId GetStatId() const
{
RETURN_QUICK_DECLARE_CYCLE_STAT(PrimeCalculationAsyncTask, STATGROUP_ThreadPoolAsyncTasks);
}