You are getting this error message because either you have used the function add_menu_page outside any hook or hooked it too early.

The function add_menu_page gets capability as a third argument to determine whether or not the user has the required capability to access the menu so the function is only available when the user capability is populated therefore you should use the function in the admin_menu hook as following.

add_action( 'admin_menu', 'register_my_custom_menu_page' );

function register_my_custom_menu_page(){
    add_menu_page(  __( 'Custom Menu Title' ), 'custom menu', 'manage_options', 'custom-page-slug', 'my_custom_menu_page' );
}

function my_custom_menu_page() {
    echo __( 'This is custom menu page.' );
}

See the following WordPress codex page for information about it.

http://codex.wordpress.org/Function_Reference/add_menu_page