In Voiceflow, the button layout setting is not global but specific to each individual step. You can set how you want your buttons to appear in the Prototype and Test Tool, using the options found in the '...' menu, under 'Buttons Layout'. There are two options:
Stacked - this will display your buttons stacked vertically, left-aligned in the chat.
Carousel - this will display your buttons horizontally, so the user can scroll through them in the chat.
If you want some buttons to be stacked and others to be in a carousel, you need to adjust the settings for each step individually. Please ensure you're adjusting the settings at the step level and not at a global or project level.