You can find the error logs in a few different places:

In Plesk

1. Login to Plesk
2. Click on "Domains"
3. Find the Domain you're looking for and click 'Open in Control Panel".
4. Click "Websites & Domains"
5. Click "Show Advanced Operations"
6. Click "Logs"
7. Here you will see all the logs for this site, you will want to click on "error_log"

In cPanel

1. Login to cPanel
2. Under the Logs submenu Click "Error Log"

SSH

If your SSH is directly to your server using Putty or another SSH client, you can find the logs at this location:

/var/www/vhosts/domainname.com/statistics/logs/