Must Use Plugins – mu-plugins help

I am creating a php code to listen for a post from a webhook.

So I put it in mu-plugin since it will execute and never be overwritten when updates happen.

I have never created a plugin, so no idea how to do it with a plugin.

My question is, if it finds a post from the webhook, then does what I need it to, then have it print the header with 200 ok, then exit, why won’t the things that wp does work, such as:

wp_create_user( $_user_name, $random_password, $_user_name );

That code causes my site to get an error.
if I put the // in front, no error. if I let it do it, I do get an error.

Also other things did not work, such as get_user_by did not work, but creating this function:

function get_user_by( $field, $value ) {
    $userdata = WP_User::get_data_by( $field, $value );
 
    if ( ! $userdata ) {
        return false;
    }
 
    $user = new WP_User;
    $user->init( $userdata );
 
    return $user;
}

that does work… so how do I do this:
wp_create_user( $_user_name, $random_password, $_user_name );

if it does not work?

Is there a better way to create some code that would work, without using the mu-plugin?

Thanks,
-Richard