Hello there,
If you are getting a red x after pasting the Auth Code, I would suggest generating a new Auth Code and pasting that in to check if it works now. If not, there 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 which plugin or theme was it and please contact the author of the conflicting software to see if they can fix this.
Thanks.