I was trying to create an archive for my custom post type (Events). I would create the archive page, but then it would load the regular WP interface, not the blocks. I looked in the forum and saw that I needed to make sure the radio button for "Show both the legacy and Blocks interface and let me choose which to use for each item I build" was checked. I did that, and I was then able to see the Edit with Blocks button in the upper right when I created my archive.
However, when I clicked the button to edit with blocks, I got the message shown in the screenshot (see attached). It basically said the archive is not compatible with the block editor and then sent me back to the regular WP interface again. I'm unclear why this is happening, especially when I can create content templates with the block editor without issue.