If you are using your own custom Abandoned Cart then you have to put dynamic code on the button link.

its added..

Is it working?

If no then put this code: {{ event.extra.responsive_checkout_url }}

Building on what @forman has highlighted, I would also recommend previewing your flow email and locating the metadata field which would highlight the checkout URL and it’s specific dynamic syntax. I recommend doing it this way as depending on the integration or event you are using to trigger this flow, the dynamic syntax may be different than what others have suggested. You can find steps to locate this information and how to reference the specific dynamic syntax for use within your emails from the About Using Event Variables to Personalize Flows article.