Recently Apple posted an announcement, according to this the intermediate certificate got expired on Feb 14th and has to be downloaded manually and should replace the older expired certificate in the System keychain.
I am unable to delete the existing certificate from the Keychain, leave alone importing the new one. How do I delete the old one and replace it with the new one?
Steps already followed: https://stackoverflow.com/a/35406327/1781918
The error message says "Operation not permitted". So you don't have the necessary access right. To solve this click on the lock in the upper left corner of the keychain and enter your password.
After you entered your credentials you should be able to delete the old certificate.
https://discussions.apple.com/thread/7357200?start=0&tstart=0
As you can see in the above link, some keys cannot be deleted while the OS is running in full mode, and you need to boot in recovery mode.
I figured out how to resolve this issue.
First, I enabled the root user in my system
In the terminal, I logged in as root user
Although, the certificate which you just deleted will appear again in the System keychain but all your other Developer and Distribution certificates will become valid again.
Make sure you set back the Keychains folder permission back to 755
chmod 755 Keychains/
I was able to delete the certificate by first viewing the
expired certificate
in system keychain and then deleting them. Then rechecking if the certificate was deleted from the certificate keychain as well. Then downloading new certificate and installing them. This might help you.