Change the config.php file for the subdomain and ip address so that npm can work correctly. Use the actual ip address of the server.
/var/www/nextcloud/config
nano config.php
Look for the below
‘trusted_domains’ =>
array (
0 => ‘localhost’,
1 => ‘server1.example.com’,
2 => ‘192.168.1.50’,
3 => ‘[fe80::1:50]’,
),