Markdown requirements, Tiki flavored, Github flavored and additional options... (confused)
You should delete that page and start again, or edit it to say editor="plain" due to the bug that was there when you created it.
The Invalid syntax type selected. Valid values: tiki or markdown. plugin on the first line is how tiki now decides which syntax and editor to use in a textarea, so this is now available in all placed (blogs, comments etc). There is some new javascript code that should hide that from you while editing and then replace it when you save, so maybe you need to clear you browser caches again to get that working?
HTH 😊