Fix the plugin please (fixes inside)

In the classes you have, in several places, hardcoded strings wp_woocommerce_cpsi, and that should be prefixed with {$wpdb->prefix} (we don’t all use default database tables).

The way you call uninstall hook is wrong. Just use uninstall.php. It’s better and won’t throw notices.

Last (I am hopeful it’s the least), you are using validate_settings_fields() method, which has been deprecated since WooCommerce version 2.6, and has no replacements.

I’m not 100% sure should I just remove

$this->validate_settings_fields();
if (count($this->errors) > 0) {
    $this->display_errors();
    return;
}

or not, but I recon you should do some sort of manual validation.

Hope this helps, and I hope you fix these issues.