I played as the dutch and it never gave me the opportunity to purchase this ship in my cities, even after researching the 3-masted ships. My city has a harbor, don't know what else I could be missing.

Just FYI: Also same with Norse Long ships. They are the "transport vehicle" or embarked naval units.

It is also worth mentioning that unlike in Civ VI (V?) these embarked units can not only defend themselves but they can attack as well.

You should probably move thread to Archives or another forum, as this is NOT A BUG.