How can I test Fast Bundle on an unpublished theme?

You can test the Fast Bundle app on an unpublished theme by following the below steps:

1- From "Online store" > "Themes", go to the “Customize” section of the theme you want to add Fast Bundle into.

From the apps, search for Fast Bundle,

Toggle on the radio box and save.

This solution should solve the issue

If you are not comfortable with making the changes yourself, you can request it via the help widget in the right bottom corner (chatbot) of your Fast Bundle admin panel or send an email directly to support@fastbundle.co. In order to set the Fast Bundle app to work on an unpublished theme, we need collaboration access to your shop. We will send the collaboration access request when you contact us. After you approve the access request we will handle it for you.