Just created a handful of points, but noticing that clustering feature is not available and there's no explanation as of why. The documentation doesn't cover this and the UI provides a dead-end.
Are you working with hosted point feature layer?
Don't you see the Cluster Points icon when you hover your mouse to the layer?
In case you don't see the icon, you need to check if you have supported point layer. Click "Show item details" to check the layer description.