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. I use the "[node:title] | [site:name]" tokens for the main image of news articles, blog posts, and similar publications. It's a quick and practical automatic ALT.
How to Improve SEO Performance in Backdrop CMS
I understand how tokens work, but not so much their practical use in image alt/text (maybe caption?) text. Could you give an example or two?
How to Improve SEO Performance in Backdrop CMS
https://backdropcms.org/project/metatag https://backdropcms.org/project/xmlsitemap https://backdropcms.org/project/imagefield_tokens These are more powerful modules, in my opinion...
How to Improve SEO Performance in Backdrop CMS