O365 is alerting you that the Symantec DLP cloud detector cert is about to expire
Please note:
Certificate renewal in all current versions should be automatically done behind the scenes.
If you are receiving this message it is likely that there is a firewall or SSL interception preventing the renegotiation of this certificate.
Requesting a new certificate will likely fail when importing for the same reason that caused it to fail in the first place.
Please see:
MPKI certificate errors in Servers and Detectors Overview for the Enforce Server
And for further troubleshooting.
DLP Cloud Service enrollment: error requesting client certificate from Symantec Managed PKI Service
Contact support in order to request a new Cloud Detection Server certificate after all firewalls have been opened appropriately.