While working with power virtual agents and making it work on Portals I found, with the help of support, that while the stacking of the options is possible, it is blocked in the backend of power apps.
My suggestion is to allow users to make the chatbot options (the blue rectangles that take you to different topics) stack on top of each other rather than force the user to scroll sideways to see the alternative options.
This will allow chatbot windows to be smaller and take up less space on the screen and it would make the chatbot easier to interact with on mobile devices and make it less likely that the user will accidentally select an option instead of scrolling to see the others.
Chatbot in portals with options side by side:
Chatbot on W3schools html tester with options stacked: