Hey @DaMaplelixir
Before you contact Klaviyo Support I would suggest to check this first. You should paste your sending subdomain into MXToolbox or whatsmydns.net. Make sure your CNAME records are actually working everywhere not just showing as verified inside Klaviyo. Sometimes Klaviyo says everything is okay before your CNAME records are actually working everywhere and that can cause problems with the activation step.
If your CNAME records look good in those tools there are two other things you should check. First you should see if the same domain is already set up as a branded sending domain in another Klaviyo account. Klaviyo only lets a domain be used in one account at a time. If it is already being used somewhere else the activation will not work. Second you should look at your DNS records to see if there is anything left over from an email service provider like Mailchimp or SendGrid. If there are conflicting CNAME or TXT records they can verify okay. Still block the final activation.
If none of that helps you should really contact Klaviyo Support. When you write to them you should tell them that you have already checked your CNAME records with a third-party tools and tried using browsers because that will save them from telling you to do those things.
I hope this helps you with your Klaviyo issue. Thank you for sharing your question in the community. If you are still having trouble you can schedule a call, with us - https://flowium.com/lp/klaviyo-agency/
Cheers,
Arpit