Some electric kettles have narrow openings that make it difficult to get a scrubbing brush inside. Fortunately, you can get them clean with some boiling water and a little citric acid powder.
According to household blog One Good Thing by Jillee, you just need to fill your kettle halfway full with water and bring it to the boil. Turn the kettle off, add a couple of tablespoons of citric acid powder to the water, and let it sit for 15 to 20 minutes. That should help take off any stubborn scaling or mineral deposits.
How to Clean Your Electric Kettle [One Good Thing by Jillee]