Hey @Fatih
If the list connection looks right, the problem is likely a technical conflict on your site. I've seen this happen when a shop has the Klaviyo "Active on Site" script installed twice, which causes a "double-fire" that the server rejects. It’s also worth checking if you’re just being rate-limited. If you’ve been testing the same form with the same email or IP address over and over, Klaviyo’s security might be flagging you as a bot. Try a totally unique email in a private browsing window to see if it clears up. Aggressive ad-blockers can also kill the Javascript the form needs to talk to the server, so try testing on your phone off Wi-Fi to rule that out.
I hope this helps and thank you for sharing your question here in the community. If you are still stuck, feel free to schedule a call with us - https://flowium.com/lp/klaviyo-agency/
Cheers.
Arpit