Trying to install subscriber-control plug in, keep getting error message. Help p

Everytime I install this pluging subscriber-control I keep getting the same following message:

Warning: Cannot modify header information – headers already sent by (output started at /home/manie98/public_html/subscriber-control.com/wp-content/plugins/optinapp.developer.1.1.9/scripts/admin.php:1) in /home/manie98/public_html/subscriber-control.com/wp-includes/functions.php on line 861

Warning: Cannot modify header information – headers already sent by (output started at /home/manie98/public_html/subscriber-control.com/wp-content/plugins/optinapp.developer.1.1.9/scripts/admin.php:1) in /home/manie98/public_html/subscriber-control.com/wp-includes/functions.php on line 862

So I created an add on domain because I thought maybe it be one of the many plugins I have installed. This add on domain I installed Akismet and thats it. I then tried to install the plugin and I get this message as well as my header showing the previous error message above:

The plugin generated 3 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.

Please advise on what I should do thanks