Drag one or more of the villagers regardless the skill to the construction area. If they continue the construction and fulfill their duty completely, the hut will be completed. Sometimes, it needs multiple drags to make them start working.
Having multiple villagers working on the project at the same time helps complete the project faster.
The more experienced the builder you have working on it, the more likely they are to continue working on it on their own. This is true of all building related projects.
This is one of a variety of ways you can help villagers gain building skills.
After the hut is completed, more unbuilt huts will appear as you approach your population cap, provided you have the necessary Technology.
Increased village population cap.
Your builders constructed a new hut! Your village can support a larger population now.