I have developed a plugin on one site and just now moved it to another. At start I had that problem in the initial site too. Saving permalinks again solved it in the initial website.
I then moved it to the new website and I am getting it again. I tried to save the permalinks settings again but it isn't helping.
I saw multiple solutions but none solved the issue.
This is how I create the custom post type:
register_post_type( 'ossp', array( 'labels' => array( 'name' => __( 'Sale Page' ), ), 'rewrite' => array( 'slug' => 'sale-page', 'with_front' => false ), 'public' => true, 'hierarchical' => true, 'has_archive' => true, 'publicly_queryable' => true, 'supports' => array( 'title', // 'page-attributes', 'editor', 'excerpt', 'thumbnail', ), ) );
If any more information is needed, tell me and I'll edit the question.