Thank you for posting this in Microsoft Q&A.
Looks like you are configuring Entra cloud sync in your environment and you are getting an error as mentioned in your above question.
Error indicates the agent is unable to communicate with your domain controller. In order to provisioning agent to communicate with domain controller, The provisioning agent must be able to communicate with one or more domain controllers on ports TCP/389 (LDAP) and TCP/3268 (Global Catalog).
Required for global catalog lookup to filter out invalid membership references
This is one of the pre-requisites that is mentioned in below article,
Let us know if you have any further questions on this.
Please "Accept the answer" if the information helped you. This will help us and others in the community as well.