Hey,
You may have to be on a Pro Standard plan with Carrd to connect it to your Convertkit form. You'll need the numbers showing in the URL when you edit your form.

You can also check this guide for reference:
https://help.convertkit.com/en/articles/2502673-connecting-convertkit-with-carrd36
If it's still not working, I recommend reaching out to the team at help@convertkit.com6 so they can check this further.