@AttainDigital is the 404 error page you are seeing like what is described in the link below:

https://wordpress.org/support/topic/woocommerce-auth-wordpress-url-fail-in-status/ ?

Please make sure permalinks are enabled. Woocom auth doesn't work otherwise

1. WooCommerce, click into the Plugins tab and select Add New at the top of the page. ...
2. On the Add Plugins page, search for Klaviyo.
3. Click Install Now.
4. Click Activate to activate the plugin. ...
5. In WordPress, navigate to Plugins.
6. WooCommerce, click into the Plugins tab and select Add New at the top of the page. ...
7. On the Add Plugins page, search for Klaviyo.
8. Click Install Now.
9. Click Activate to activate the plugin. ...
10. In WordPress, navigate to Plugins.

Hope this is going to help

@AttainDigital  I received confirmation that the problem was solved in the internal Klaviyo ticket. Do you mind sharing with the community what was changed to allow the connection to Klaviyo to work?

Hi there, 

I am trying to install Klaviyo on WooCommerce. I have installed the app and trying to connect accounts.

When I go to connect accounts, it displays a 404 error page.

Here is the link URL on the ‘Connect Account’ button:
https://www.klaviyo.com/integration-oauth-one/woocommerce/auth/handle?url=https://development.brstdev.com/klaviyo

Here is the actual URL when clicked:

https://development.brstdev.com/klaviyo/wc-auth/v1/authorize?app_name=Klaviyo&user_id=Y25GtF_WdDNXc&scope=read_write&callback_url=https%3A%2F%2Fwww.klaviyo.com%2Fintegration-oauth-one%2Fwoocommerce%2Fauth%2Fsave%3Fdomain_url%3Dhttps%253A%252F%252Fdevelopment.brstdev.com%252Fklaviyo%252F%26timestamp%3D1662445386&return_url=https%3A%2F%2Fwww.klaviyo.com%2Fintegration%2Fwoocommerce%3Fauth_redirect%3DTrue%26domain_url%3Dhttps%253A%252F%252Fdevelopment.brstdev.com%252Fklaviyo%252F