I fixed this issue on mine (not using XMLRPC, but it should still work, since I had the same message when trying to use another plugin to create posts).
Basically, you have to give yourself permissions to post.
Go to the “Roles” menu. Let’s say you have your site set up to restrict by Categories. So click on “Categories” on the left menu.
Choose these options.
1) Dropdown: choose “Assign for selected categories”
2) Select your user name checkbox
3) Select nothing here
4) Select “Post Author” for all categories. If you have a lot of categories, you can do this with a “select all” checkbox at the bottom.
5) Press “Update” in the upper right.
So you have given your user name / account permissions to be a post author in all categories (assuming that you are an admin and you wan to be able to post in all categories).
The idea is the same if you are restricting by posts or by pages or whatever else – give your account permission to be an author and that should fix it.