Occasionally, the Office 365 portal will report that there is an issue with the password sync from the on-prem AD server.
Prerequisites
You will need access to an account that has administrative access to the server running Azure AD Sync
Process
First, you should make sure that the latest version of the Azure AD Connect Sync utility is installed on the server. You can check the latest version on the Azure AD Connect: Version release history page.
If that does not resolve it, you will want to Troubleshoot password hash synchronization with Azure AD Connect sync.