If after you have set up your WordPress blog accounts inside WikiLink Jacker the application is unable to post an article, or retrieve article categories from your blog, then it's possible that you have a plugin, such as a security plugin, that is locking down the XMLRPC function on your blog.
Some security plugins do this to protect your blog from hack
attempts, but if they shut that down, then the app will not be able to
use the remote posting option. (That's what XMLRPC does.. it allows
remote access to your blog to post and fetch categories etc.)
An example of a frequently used blog security plugin that is known to block the remote posting features of WordPress is WordFence. However there are many other security plugins so you'll need to review your blog plugins to see if you may have another security plugin installed which could be the cause.
So to debug it, go through your plugins one by one and deactivate them, starting with the security plugins you have installed and see if turning off any particular plugin corrects it.