Quoting this answer: https://community.klaviyo.com/integrations-39/magento-2-4-1-integration-issue-2147?postid=7443#post7443
I have a question to "how to solve that?"
Because I am also facing a similar issue while integrating.
==ERROR MESSAGE==
We ran into an error There was an error while integrating your Klaviyo account with Magento 2. Below are the details about your error
Klaviyo is unable to connect to Magento Server for OAuth Handshake.
Url tried was: None
Please ensure that both /oauth/token/request and /oauth/token/access are accessible
So if you can help me, it will be very much useful.