When i go to the Alert Policies page in the S&C Center, I'm seeing "You only have access to certain system policies due to no valid subscription for Office 365 Analytics alerting detected. To get the full experience or create custom alert policies, a valid subscription is needed." I don't think that any of my subscriptions or licenses on this account have changed, and I can't figure out what this is referencing.
Can someone provide me some insight about what might be causing this?
That's the "advanced" version of the activity alerts functionality, which requires E5 or Threat Intelligence. If you ask me, that whole section of the SCC is a total mess. We first had the "regular" activity alerts, included with no additional licensing, which have since been de-emphasized to the point that you can only access them via a link on the Dashboard page (https://protection.office.com/#/managealerts). They introduced the "advanced alerts" later on, part of CAS. And then we got the "alert policies" which are basically still activity alerts, but with a fancier UI and signals/conditions.
Alert policies require an Office 365 E5 subscription for your organization. Alternatively, you would need an Office 365 E3 subscription with an Office 365 Threat Intelligence or Office 365 Advanced Compliance add-on subscription to use alert policies. Note that an Advanced Compliance subscription doesn't support alert policies related to malware attacks and phishing messages.