You can’t use an input_boolean, nor a switch, only binary_sensor.
As noted in bold in the first post:
Please note that the Alexa component will only recognise binary sensors that have one of the following device classes: door, garage_door, opening, window.
You must use a binary_sensor entity, with one of those class types (or the CONTACT_SENSOR display category). But (as noted in the first post), it needs to be a binary_sensor.