Hello @Sophie Bennett,
Building off of @retention’s wonderful suggestion, can you let us know if the form you are using is a Klaviyo built form or one provided to you as part of your Shopify theme?
If the form was a provided to you as part of your theme, can you confirm if you’ve updated the form’s hidden input group tag with <input type="hidden" name="contact[tags]" value="newsletter">
and enabled Accepts Marketing in your Shopify integration settings? A refresher on how add this hidden input group tag can be found in the Sync Your Shopify Newsletter Subscribers to a Klaviyo List subsection of the How to Integrate with Shopify article.
I would also recommend taking a look at some similar Community posts, such as the ones below, to see if any of the solutions provided in them can help you resolve your issue as well
Hope this helps!
-David