can't build dockyard/harbor. Why?

[MasLegio]

Is this a bug?
Cant build dockyard even though I have several sea squares

[Lemurian1972]

1) Did you research the correct Technology?
2) Are those squares within 3 spaces of your city center?

[paugus]

3) Do you have the population to support another district in that city?

[donald23]

Do you have a place to build it, that has no cliffs? (as far as I know you cannot build a harbor next to only cliffs)

[Azunai]

if you post a screenshot of that city, i'm sure someone can tell you why you can't build a harbor.

[Sable]

There is also a bug.

For all other districts, if you can build that district but it's not yet within the city boundary, the district appears on the build menu and selecting it gives you the option to purchase the tile and then build it.

Not so with harbors. With harbors the menu item is not available until you actually own the tile, so you have to purchase the tile separately first.

This may not be what is happening here as you say you have sea tiles, so check what everyone else has said first as it's good advice.