One of the most frequent reasons why /install shows an 'Under Construction' page is that the shop URL is set incorrectly in the affiliate area Other Settings are set incorrectly. The URL should be set as http://www.yoursite.com/ or http://yoursite.com/ (if it's without 'www') - as below:
As there is no way to correct the URL in Other Settings once you've started the installation already you should edit the shop data/config/config.php and correct the URL and domain there as below:
$config ['site_domain'] = 'www. yoursite.com';
$config ['site_web_address'] = 'http://www.yourd.com/';
$config ['site_domain'] = 'yourdomain.com';
$config ['site_web_address'] = 'http://yourdomain.com/';
(depending on if there is or there is no www for the domain and URL).
If you have more questions feel free to contact our affiliate support by submitting a ticket request.