When I try to RDP into a windows server 2012 with a non-admin account, I get the following message : 

To log on to this remote computer, you must be granted the Allow log on through Terminal Services right. By default, members of the Remote Desktop Users group have this right. If you are not a member of the Remote Desktop Users group or another group that has this right, or if the Remote Desktop Users group does not have this right, you must be granted this right manually

To correct this issue, first, log in as an administrator on the NComputing host and click Start and then click Run. 

Type in "gpedit.msc" 

Click "OK" 

When the Group Policy Object Editor appears, navigate to: 

Computer Configuration > Windows Settings > Security Settings > Local Policies > User Rights Assignment 

In the right-hand pane, locate the entry named "Allow Log on through Terminal Services" and double-click on it. 

Click "Add User or Group.." 

Find the user or group to add and apply the changes.