Hello @AdamS7255 ,
In the URL at the top of the page, does it may have '.myshopify.com' twice (for example: https://companyname.myshopify.com/.myshopify.com/admin/oauth/authorize?client_id )? If it does, try deleting the second ".myshopify.com" portion and try the URL again. If issue continues, also try removing the / at the end of the URL
Another potential troubleshooting step is to go in Shopify after installing the integration, Apps > Constant Contact and log in to complete the connection. In CTCT, on the Shopify integration details page, make sure that the Store name appears. If it does not appear, then the integration hasn't been connected properly with the right permissions.
If you continue to get the 404 error, please let us know if the issue is persisting across multiple browsers / in Incognito browsing. Additionally, if your Shopify site isn't live yet, that could also be causing the error.
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