Firstly, check the syntax of the apache file with the following command:
You can either fix these manually, or preferably, rebuild your configuration using cPanel's tool. Using cpanel's method, backup your httpd.conf, and run /scripts/rebuildhttpconf
Then check the syntax again and try your change if that resolved the issue.
Please note if you've been making manual changes to the httpd.conf without using cPanel's tools or templating systems, before trying anything, backup your httpd.conf. If you're on cPanel 11 and havent' been modifying things using cPanel's tools, it will overwrite your changes and you could lose all your configuration data for apache.
If rebuiding the apache configuration didn't fix the issue, I would check the logs for cPanel, the error log is located here:
Be careful not to restart the cPanel service, or try your change again to make sure its in the logs, as restarting cPanel wipes out the error_log (one of a thousand /facepalms you run into with cPanel).
Failing anything specific there, I've not seen that specific error, but there are some steps you can take to hopefully 'voodoo' cpanel into fixing it, including running /scripts/rebuildcpusers, followed by /scripts/rebuildhttpdconf.