If you are trying to install Elgg and getting this error, then this post is NOT for you. If you are developing a plugin on a working Elgg system and getting this error message, then you have come to the right place.

When Elgg is unable to locate the action file, it give the following error message:

The requested action (plugin/file) was not defined in the system

First of all, make sure that your action file is registered in the init function of start.php:

register_action("myplugin/actionfile", false, $CONFIG->pluginspath . "myplugin/actions/actionfile.php", true);

Then make sure that you action file reside in the correct location.

myplugin/actions/actionfile.php

is referenced by

$vars['url'] . 'action/myplugin/actionfile