When you initially sign up for Intune, the service issues you a certificate. When the certificate renews, an alert will pop into the Intune console letting you know to go download a new Windows PC Management client software package, and to use that package for any new PC enrolments. This new client software package is only used at enrolment, so your existing managed PC’s continue to be managed by Intune. But the new PC enrolments – those are the ones that won’t enrol without a new Windows PC Management Client software package.
If you’re an Intune Admin, you’ll see an alert in-console:
The error your end user will see when they try to enroll in Intune looks as follows: