Ash here from the Shopify team. Welcome to the Shopify Community!
This is definitely a strange sounding issue, and I'm happy to help out. I see you mention trying in multiple browsers already, and I also see from your screenshot that you've recently tried in FireFox. Could I get you to switch over to Google Chrome and try these troubleshooting steps please?
Could you also please tell me if this is occurring with any other apps you try to install, or is it only Klaviyo? You can try installing a free app to test this, and can then delete it after if it installs successfully but you don't wish to use it.
Looking forward to hearing back so we can get this issue resolved. Please reply to this post with your answers to the above so I can continue to help out.