Hi there @Seller_mTPrblWCg0CQq,

Thanks for reaching out with your difficulties. I'm sorry you are having a difficult time enabling customizations. I would recommend the following.

1) Ensure you are using a browser that is the most compatible with Seller Central. In general I recommend Chrome or Firefox.

2) Ensure your browser is up to date and has the most recent updates.

3) Clear your cache and cookies to clear any aged backend data.

Once you have completed these steps attempt to resubmit your file. If it fails again provide your most recent screenshot to Seller Support in a new case and share the Case ID here and I will see what next steps for troubleshooting are available. Thank you for your understanding.

-Glenn