I’m not sure I understand your intent.
Classic Editor is the old editor, which is now a plugin and restores the old editing experience. Once you install the plugin, you can set it as the default editor for all new and existing posts.
Classic Editor Block is the old editor available as a Gutenberg block and displayed inside Gutenberg. It is just one of several blocks.
Now your question was setting the “Classic Editor Block” as the default, which is why my little brain is confused.
Did you mean Classic Editor instead?
If you really meant having the Classic Editor Block be the default block that is open when you launch the editor (instead of the paragraph block), at this time that will require custom coding. Please see this Stack Exchange Q&A.
