With Android, third-party applications have always had problems with access to certain parts of the directory tree, and the newer the application, the more limited the permitted area of the directory tree.
Google, the makers of Android, only trusts its own applications and gives them the most far-reaching access rights. In addition, the providers who equip their hardware with Android as the operating system, Google also trusts their applications and grants them far-reaching access rights across application boundaries.
Furthermore, Google seems to grant the applications of a few big players in the software sector more rights in the file system, examples known to me are Microsoft and Adobe, whether money is involved or the old story of one hand washing the other is unknown to me. Unfortunately, Google does not (or no longer) trust the applications of smaller companies and individual developers who offer their apps via the Play Store and prevents them from exchanging and accessing files across application boundaries, and even from Android’s own file manager you cannot start files that then start in these “third-party applications”.
To open images in Krita, you have to do this from within Krita, as @CrazyCatBird has already recommended.
If you want to read a bit more about Googles practices, you may want to read this (my view):
If you want to search yourself for such things, you have to use Bing or other non-Google search engines, Google pushes the search results on this topic very far back in the list. The best places to research this topic are computer and smartphone/mobile computing magazines on security, it’s so sad you have to laugh at it again.
Michelist