Some folder can’t be selected for the database, but some can, such as
You cannot create a database in any cloud-synced location.
But as you see, the folder named
Obsidian in iCloud is highlight.
I can create a database in this folder.
Well, don’t. DEVONthink is designed to use a local database only. Placing the database on a server, whether cloud or semi-local, will cause data corruption at some point. The only exception is for a wired NAS.
Creating, I guess is one thing. Using without much failure (due to how syncing works and how DEVONthink not designed to do that, and is documented) is another thing.