It is possible to put an animal flap into a door panel made of upvc, or a double glazing glass sealed unit however the best choice is to buy a new glass pane that has a premade hole installed. Follow the directions that should be included with the kit for https://cat-flap-in-upvc-door-pan09096.pointblog.net/adding-a-cat-flap-to-your-bifold-doors-72851454