Most probably someone who uses the same hosting company was abusing LinkedIn. As the result, LinkedIn blacklisted your hosting company’s whole IP range.
Please just follow instructions from the message and it will work.
There are only two ways to solve this.
1. Please find a working proxy server and test it in your browser. Login to LinkedIn from your browser configured with proxy and make sure you get to your account. Please solve all captchas and/or enter all codes from SMS and/or email.
Enter the same proxy to the plugin. Submit the test post and plugin will get all necessary saved login info.
Remove the proxy and send another test. Plugin will login to LinkedIn with saved info and LinkedIn will mark your server IP as “Safe to login to that account”.
It will work fine after that.
You can find option “Proxy Settings” in the “Advanced” tab. Please use format “IP:Port” and “Username:Password”.
2. You can use the hosting company who’s clients haven’t tried to abuse LinkedIn previously and who’s IPs are not banned there.