The reason you receive this message from the website is that you are navigating to https://www.[client ID].gogiftsonline.com, rather than https://[client ID].gogiftsonline.com.
Example: https://www.giftsonlineisgreat.gogiftsonline.com will cause the error to occur, but https://giftsonlineisgreat.gogiftsonline.com will not.
To move past the error, remove 'www.' from the address and hit Enter. This should direct you to your GIFTS Online site.