Hello @Vinn,

Welcome to the Klaviyo Community!

I would suggest making sure you’ve set up and enabled your Alphanumeric number. As mentioned in our Getting Started with SMS and MMS for the First Time Help Center article:

If you plan to send in the UK or Australia, edit your alphanumeric sender ID to indicate your brand. Setting this up is key, as you will not be able to collect SMS consent from people in these countries until you have an active alphanumeric sender ID.

I’ve previously explained this behavior in the Community post I’ve linked below:

I hope this helps!

David