Hi,
We shouldn't have an error when typing just wikitext (and neither page name nor rev id): wikitext is sufficient for testing some code.
Instead, when testing wikitext, we have to put a random page name to make the submitting work.
Topic on Extension talk:TemplateSandbox
A page title is required so magic words like {{PAGENAME}}, Scribunto modules that reference the current title, and so on have something to refer to.
Ok, I understand. Thank you!