Curtis Wilson Cost Gallery review for Squirai: Promotion Popup

I am using Shopify Dawn, which is supposed to be compatible. When attempting to edit a popup template, I receive the error message "tiny MCE is in read only mode" with a prompt to upgrade our billing plan. But clicking to learn more or upgrade billing returns a 500 server error. I've tried clearing cache and used multiple browsers. Without the ability to customize templates, this app is useless.