Apologies for not being able to add a comment to the talk page.
To deter spammers, a requirement of someone with a new Moodle Docs account is that they must make at least one page edit before they are allowed to create a new page. Normally this works well, since newcomers are likely to edit a page before thinking of creating one. You have highlighted a situation where it doesn't work well. As Moodle Docs now has other anti-spam measures in place, we're going to try removing the page edit requirement.
Returning to https://docs.moodle.org/31/en/Chemistry_notation_using_mhchem I realised that the message at the top of the page was not applicable and so have removed it. (The message was for use when drafting developer documentation and was created at a time when we included user and developer docs in one wiki.)
If your patience with Moodle Docs is not totally exhausted (quite understandable), please feel free to edit https://docs.moodle.org/31/en/Chemistry_notation_using_mhchem directly. Otherwise, if you'd prefer to leave a comment on the talk page, if you make a quick edit to https://docs.moodle.org/31/en/Sandbox first, you'll find you are then able to create the talk page.