Hello there,
I would start off by asking if there is a green check mark or a red X mark when you enter the API key. Also it might be a conflict in between Optin Cat and your currently active theme or other active plugins. To search for conflicts, please follow these steps.
1. First please deactivate all plugins except Optin Cat and see if the issue persists.
2. If the issue is resolved, reactivate your plugins one by one to see if the issue returns.
3. If no conflict is found among your plugins, try reverting to a default WP theme like ‘Twenty Sixteen’, or ‘Twenty Seventeen’, and see if the issue is resolved.
Head here for detailed instructions:
https://fatcatapps.com/knowledge-base/search-for-conflicts/
If you find a conflict with a plugin or theme, please let us know what was causing the conflict and also try to contact the theme or plugin author of he conflicting software to see if they can fix it on their end.
Thanks.