In reply to Steve.

Steve,
You need to enable FSE (full site editing) (https://make.wordpress.org/core/2020/12/10/status-check-site-editing-and-customization/) in order to use the query block (a few other other blocks become available as well).

Note that enabling FSE will cause changes to your website and theme; so I would recommend to only use it on a testing environment where you feel comfortable experimenting on.

There is a proposed solution to decouple the query block so it doesn't require full-site editing see https://github.com/WordPress/gutenberg/pull/27822

Decoupling the query block from the full site editing experience would be extremely advantageous. Theme developers and users can begin using it, share feedback, and find its faults (and advantages) with the architecture of the block.

In fact, I'm wrapping up a project this week and would have used the query block if it was more polished; I didn't feel comfortable to put live into a somewhat critical website as soon as next week.