Hi Cynthia,
Thanks for reaching out and thank you for using BoldGrid Post and Page Builder for WordPress!
In most cases you are indeed correct, you are able to set your preferred editor to use the Post and Page Builder by default. However the theme you are using is one of the new block themes used with the Full Site Editor. This is a relatively new addition to WordPress and is built to work with the WordPress Block editor.
Unfortunately the Post and Page Builder is not compatible with the Full Site Editor or the block themes that utilize it at this time. While you can still use Post and Page Builder for individual pages or posts it’s recommended to use the WordPress Block editor to avoid unexpected errors or issues.
I hope this helps. Please reach back out if you have any other questions for us.