Answer #1:

Usually, It's Access Denied error. Some Virto components add Web.config modifications, but SharePoint 2010 could not save them. 

Access denied when deploying a timer Job or activating a feature from SharePoint 2010 content web application

 Answer #2:

You should use STSADM utility from SharePoint package to activate feature, run command line as administrator
"c:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\BIN\STSADM.EXE"  -o activatefeature -id <product featureid> -url <your sps site url> -force

Answer #3:

You should use SharePoint Power Shell command to activate feature
Enable-SPFeature <feature_name> -Url <site>