You may receive the following error when you try and perform a backup through the cPanel >> Backup interface
"The system failed to create the requested backup.
A backup is already in progress with PID 2038. at
/usr/local/cpanel/Cpanel/API/Backup.pm line 234."
This error will display when there is already a backup running for that user, and this is just letting you know that you can't have multiple backups running at once.
You can run the following command on the system to confirm the process ID mentioned in the example is still running, which is process 2038.
ps aux | grep 2038
If that is still running you'll want to let it complete before trying to create another backup.