When you save the credentials, the system tries to test the API by checking that the 'namecheap.com' domain exists by POSTing an API request to Namecheap with your credentials. However, Namecheap did not return an 'OK' response, or the system could not interpret the response. The module requires the libxml php extension to be enabled on the server, so you may want to double-check that it is.
Yes, but for custom fields, i want client to fill them in the signup form, but disable it from editing . Not sure if this is your attention .
a simple ; custom client field gender , i want client to be able fill it in the signup form, but he can't edit it in the profile page.
This isn't a bad idea, disabling the editing of certain fields like names or company names could prevent accounts from being so easily sold to someone else. We'd probably use it internally ourselves.
The restricted packages are currently only orderable by admins from the admin interface. However, v4.3 addresses both of your concerns via CORE-718. That is, clients will be able to order restricted packages through the order plugin, and the order plugin will only display the package group if it contains at least one restricted package the client can order.