I'm mystified.

I picked the Norse in order to be able to build the Langskip.  I researched Seafaring Master, and I am now offered Cogs as a unit to build, but not Langskips.  I see no special resource requirements.

What am I missing?

The Langskip is a transport ship and not a combat/buildable one. So this unit replaces land units that embark to travel across water tiles, like the earlier Transport Galley.

Thanks!