Quickly creating an issue for this; proper issue summary will follow.
https://www.drupal.org/docs/8/upgrade/upgrade-using-web-browser#pre-upgr...
If the database credentials to your source database are correct, the upgrade review page will appear as illustrated in the screenshot below. This page shows a summary of the upgrade status for all installed modules on the old site.
Latest iterations use this help text instead:
The new site is missing modules corresponding to the old site's modules. Unless they are installed prior to the upgrade, configuration and/or content needed by them will not be available on your new site. Read the checklist to help decide what to do.

Recent comments
Hi Kevin I am interested assisting you developing a theme by cloning feature from existing WordPress website. Please let me know your suitable time to discuss further...
Create a theme from existing website
I've updated the Zulip link in both places I found it. No need to post again, unless you have something new to say. We'll pull together feedback from all the sources.
Backdrop CMS Core Priorities
Should we post here again, what we posted over there? Or would that unnecessarily duplicate things? The link to a Zulip thread in this initial post leads to an internal one, but there's...
Backdrop CMS Core Priorities