What does "Contact group admin to turn on subscriptions" mean?

When you invite a Team/Group to a meeting in Outlook (have the Team/Group name in the "Required" field), the meeting invitation will be sent to the Team/Group email and end up in the Team/Group calendar.

If the Team/Group has been configured to not forward invitations to the members themselves, then you may see the following message: "Team members are not getting calendar events based on group settings, contact group admin to turn on subscriptions."

This is not an error message but for your information, the Team/Group owner can change the Team/Group settings so that invitations are forwarded to individual members calendars.