get_permalink() is only really useful for single pages and posts, and only works inside the loop.

The simplest way I've seen is this:

global $wp;
echo home_url( $wp->request )

$wp->request includes the path part of the URL, eg. /path/to/page and home_url() outputs the URL in Settings > General, but you can append a path to it, so we're appending the request path to the home URL in this code.

Note that this probably won't work with Permalinks set to Plain, and will leave off query strings (the ?foo=bar part of the URL).

To get the URL when permalinks are set to plain you can use $wp->query_vars instead, by passing it to add_query_arg():

global $wp;
echo add_query_arg( $wp->query_vars, home_url() );

And you could combine these two methods to get the current URL, including the query string, regardless of permalink settings:

global $wp;
echo add_query_arg( $wp->query_vars, home_url( $wp->request ) );

The way of getting the current url should also work in Wordpress 6. $wp->request gives you the path structure of the url (like /parent/child ). To pass the whole url as the second argument, you can use the following code:

global $wp;
$current_url = add_query_arg( $wp->query_vars, home_url( $wp->request ) );

Moreover your problem might be, that you did not wrap your code inside a function, and the data you need is not there yet. You need a hook that fires after the data is available.

But

If you are ok with using another way of getting the current url, I would suggest:

function getCurrentUrl() {
    $protocol = is_ssl() ? 'https://' : 'http://';
    return ($protocol) . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];
}

$currentUrl = getCurrentUrl();
echo $currentUrl;

In case anyone else needs this. Below code should get the exact URL including parameters.

home_url($_SERVER['REQUEST_URI']);

You can echo or return it depending on your use case. e.g.

echo home_url($_SERVER['REQUEST_URI']);

or

return home_url($_SERVER['REQUEST_URI']);