As of March 1, 2023 in LinkedIn Recruiter, Microsoft integrations only work with cloud deployments. Microsoft will no longer allow Hybrid Deployments to access calendar and mailbox APIs.
"Hybrid" means that exchange runs on a server that's physically present at your office and that server has a connection to Microsoft's servers to support use of the API. "Cloud" means that everything runs on Microsoft servers.
Microsoft support for mailboxes
The End of the REST API for On-Premises Mailboxes Preview