First - is your theme OS .20 compliant?
When Pro Blogger loads it will display a different interface depending on if your theme supports the new architecture or not.
If you see this screen - then you are still on a vintage theme and will need to follow the normal install process
If you see this screen - then your theme is OS 2.0 compliant but running with a vintage install. You can upgrade to using the OS 2.0 part only by clicking the button. For full detail check this article here
Please note - that as of the 1st of January 2024 - we will no longer be able to support pure vintage installations. This is due to changes in Shopify that prevent apps making direct updates to the theme assets.
For all articles related to vintage installations - you can find them here