Have you been made an owner of the management group(s)? This is a parent level above the subscriptions so even if you are a subscription owner you will not have owner access on the management group(s). The creator/owner of the management group(s) would need to make you an owner of the management group(s) if you need to make changes to these.
This article explains all of this in more detail:
https://learn.microsoft.com/en-us/azure/governance/management-groups/overview