If you're unable to add a block in the Theme Customizer, it's likely that your store is using a Shopify 1.0 theme, which does not support app blocks or advanced customisation options, such as switching the chat interface from floating to embedded.
Shopify 2.0 themes, introduced in June 2021, include support for app blocks. To check your theme version:
If you're using a Shopify 2.0 theme and still experiencing issues, please contact us at support@clevertar.com - we're happy to assist you further.