Get Page not Found or Forbidden

Just bought your Video Grid Plus Lightbox for $18 dollars, uploaded the zip file to my plugins, activated it, added a grid then tried to manage videos by opening a Youtube video, pressed the add new button at the top of the screen, added the link, clicked to get the thumbnail, the rest of the text fields seemed to be automatically populated. When I went to save it, it came up with Page not found on permalinks set to post name. When I changed it to plain it then came up with Forbidden “You don’t have permission to access /wp-admin/admin.php on this server.”??

What can I do to make this work? Do you provide a refund if it does not work?

https://wordpress.org/plugins/video-grid/