OSCAP doesn't work for remediation on Ubuntu 18.04 install
Have you executed the command with elevated rights?
OSCAP as a product makes various changes to protected files, which means you need to execute the command as sudo.
Edit: Since you replied with the whole command and the whole error message, the most probable cause is that there is no fix yet supplied for that specific rule, you can try ignoring that rule from running and that would help you generate the remediation.