There is no such thing as a DHC-6 Type Rating, at least not in Canada or the United States.
In Canada it is not a Type Rating because the Twin Otter is not type certificated as requiring a minimum of two pilots, nor is it a "high performance" aircraft.
In the United States it is not a Type Rating because it is not a jet, nor does it have a MGTOW of greater than 12,500 lbs.
Flight Safety is expensive.