On some wordpress version, we can not create new post because of error when update wordpress code.
We can fix that by:
1. Go to https://build.trac.wordpress.org/browser/trunk/wp-includes/js/tinymce
2. Download wp-tinymce.js.gz and tinymce.min.js
3. Upload/replace these two files in your /wp-includes/js/tinymce folder
4. Empty your cache
Maybe it will work for you !