When you want to link JIRA with Confluence by adding a page > Insert > Insert JIRA Issue you got this misleading error:

There were errors creating an issue in JIRA. Some required fields may not be available on this form. Try creating this issue in JIRA
You are not logged in, and do not have the permissions required to create an issue in this project as a guest. To create an issue first log in

We reproduced the issue and found out that it is because there are some mandatory fields that are not showing up when creating a new issue through "Insert JIRA Issue".