When working with the CentOS 7 operating system, you may encounter the "Failed to start session" error when attempting to log in through the graphical user interface. This error can be caused by a variety of factors, including user permission issues, damaged configuration files, or missing dependencies. Here are several steps to help you identify and resolve this issue.

1. Check the Logs

The first step is to check the system logs to pinpoint the exact cause of the error. Log into the system via the terminal (Ctrl+Alt+F2) and use the following command to view the logs:

journalctl -xe

Look for messages related to "Failed to start session" or the graphical environment. These messages may provide clues as to where the problem lies.

2. Fix User File Permissions

One common cause of this error may be incorrectly set user file and directory permissions in your home directory. To fix them, you can use the following commands:

sudo chown -R $USER:$USER $HOME

This command will change the owner of all files and directories in the home directory to the current user and their group.

3. Reconfigure the Graphical Environment

If the problem persists, you may need to reconfigure or reinstall the graphical environment. For CentOS 7, the GNOME desktop environment is often used, and you can reconfigure it with the following commands:

sudo yum groupremove "GNOME Desktop"
sudo yum groupinstall "GNOME Desktop"

This will remove and then reinstall the GNOME Desktop Environment, which may resolve the issue.

4. Check and Install Missing Dependencies

Another possible solution is to check and install any missing dependencies for your graphical environment. This can be done using the command:

sudo yum deplist gnome-shell

This command will display all dependencies for gnome-shell and help you identify if any are missing. If you find any missing dependencies, you can install them using the yum install command.

5. Create a New User Account

If all previous steps fail, the problem may be specific to your user account. Creating a new user account and attempting to log in with it may bypass this issue:

sudo adduser newuser
sudo passwd newuser

Then try to log in using the newly created account.

This series of steps should help you resolve the "Failed to start session" error on CentOS 7. It's important to note that the exact cause of the problem may be specific to your environment, so it may require further troubleshooting based on the information gathered during these steps.