It is an old site made before the new block editor arrives. Now it’s running WP 5.0.11, and I do not want to make an upgrade right now for various reasons.

The default post types are using the new block editor, but the custom post type still use the old TinyMCE and I even cannot insert images to it. The image problem wold not be a problem if I could switch to the block editor witch works.

Thank you in advance.