try run laravel admin ^1.6 in subfolder

laravel_admin_helpers.php

<?php
// var_dump(function_exists('admin_url'));
if (!function_exists('admin_url')) {
    /**
     * Get admin url.
     *
     * @param string $path
     * @param mixed  $parameters
     * @param bool   $secure
     *
     * @return string
     */
    function admin_url($path = '', $parameters = [], $secure = null)
    {
        if (\Illuminate\Support\Facades\URL::isValidUrl($path)) {
            return $path;
        }

        $secure = $secure ?: (config('admin.https') || config('admin.secure'));
        return url(custom_admin_base_path($path), $parameters, $secure);
    }
}
if (!function_exists('custom_admin_base_path')) {
    /**
     * Get admin url.
     *
     * @param string $path
     *
     * @return string
     */
    function custom_admin_base_path($path = '')
    {
        $prefix = '/'.trim(config('admin.url_prefix')).trim(config('admin.route.prefix'), '/');
        $prefix = ($prefix == '/') ? '' : $prefix;
        return $prefix.'/'.trim($path, '/');
    }
}