I have the same exact problem when trying to activate the plugin.
I have a network of 3 sites and when I try to activate it network wide or by site I get this error and this message:
Notice: Undefined index: master_site_setting in /home/shzion/public_html/wordpress/wp-content/plugins/multisite-post-duplicator/addons/restrictSites-mpd-addon.php on line 325
The plugin generated 1260 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.
On the pages/posts editor I get the same error for each site that I can duplicate my page/post.
I have not tried to duplicate them but this is not usable right now for me.
I’ll be glad if you can help us to solve this problem because this plugin is the only one that works with ACF and meta data without paying an extra $100
