Hello and Welcome to LinuxQuestions,
First of all, this is only an emergency to you, not to us. The error is pretty clear, you don't have any template definition for 'linux-server' and you're using it in your configuration file. So follow the documentation and create a template in the correct file so that Nagios can use it. A template in Nagios is used so that you don't have to retype repeating variables / values for each and every host to which you want to apply those. Check in your Nagios directory structure for a file called templates.cfg (nagios/etc/objects maybe?). That one holds a lot of examples. Also check in your nagios.cfg if you're including the templates file.
Looking forward to your participation in the forums. Have fun with Linux.