Verify that you've met the multi-forest hybrid deployment prerequisites as described in the TechNet article. These requirements ...

Verify that you've met the multi-forest hybrid deployment prerequisites as described in the TechNet article. These requirements include configuring directory synchronization with %BRAND_OFFICE_365_SHORT%. Typically, only one forest needs a directory synchronization server installed. That server should run %BRAND_AZURE_SHORT% Active Directory Connect (%BRAND_AAD_CONNECT_SHORT%) to configure synchronization. If a two-way trust isn't configured between the forests, you should consider deploying Active Directory Federation Services (AD FS) to enable single sign-on.