Recreating the bug: a villager is assigned to build an Ovoo but must fight a wolf on the way. Or even if you interrupt the villager and move them as they’re walking to the ovoo.
The Ovoo outline will be on the stone but right-clicking will show a NO symbol. If you try to select an Ovoo from the menu and click on the stone an error message says, “Must be built on a stone outcropping.”