The following article has been up on the KB for a long time now with no real solution
https://support.symantec.com/en_US/article.TECH249528.html
Situation
When attempting to install iOS profile on iOS11x you receive a Simple Certificate Enrollment Protocol error on your mobile device.
Environment
iOS 11x
Mobile Device Management
Error
- SCEP Server has returned an invalid response or
- SCEP Server has returned a nonvalid response or
- Profile Installation Failed, The SCEP server returned an invalid response.
Cause
iOS 11 version has a SCEP server Certificate synchronization issue on the iOS operating system. Symantec cannot provide any resolution for this issue.
Solution
Submit a bug with Apple for more visibility of the issue.
Check Blue Coat Trust Page for any updates.
Another workaround is to wipe the device but we are yet to try this one out
https://discussions.apple.com/thread/8652082