License is invalid

Hello,
New-be here. I really like the functionality this plugin advertises and it may be that I do not understand but I would like to hide the Previous/Next links at the bottom of the post page.

None of the options seem to do this and I searched the Settings and the license activation states in red “License is invalid” so I clicked the Activate (with no key) and get an Error “Invalid Plugin or theme(2)” I thought it might be our domain firewall (inside corporate intranet site) issue?

So I went and purchased the Pro plugin which when installed locked up the site until I deleted the plugin pro subdirectory. Can you help me figure out what the issue might be? Thank you.