Go to "Team & Analytics" section on the skillboard.
Go to the "Insights" section.
Scroll down to the "Skills that nobody has". Here you can find skills that weren't assessed during the skills review.
Hover the mouse over the skill and click the edit icon to open the context menu.
In the context menu you can:
Add a skill to a specific employee's "Development Plan".
Add materials inside the skill
Delete the skill from the skillboard.