The health agent can fail to register due to the following possible reasons:
The agent can't communicate with the required endpoints because a firewall is blocking traffic. This issue is common on web application proxy servers. Make sure that you have allowed outbound communication to the required endpoints and ports. See the requirements section for details.
Outbound communication is subjected to a TLS inspection by the network layer. This causes the certificate that the agent uses to be replaced by the inspection server/entity, and the steps to complete the agent registration fail.
The user can't perform the registration of the agent. Global Administrators can by default. You can use Azure role-based access control (Azure RBAC) to delegate access to other users.