You cannot move users from a different AD domain in hybrid Skype for Business Server to Skype for Business Online from Control Panel
In a Microsoft Skype for Business Server 2015 hybrid environment that has a multidomain Active Directory Domain Services (AD DS) topology, a user from a child AD domain cannot be moved to Skype for Business Online from Control Panel. The move operation fails and returns the following error message:
Cannot find user in Active Directory with the following SIP URI: <SipAddress>.
By design, the scope of the user query for hosted migration from Skype for Business Server Control Panel is limited to the AD DS domain of the Skype for Business servers. During the move operation, Control Panel runs the Move-CsUser PowerShell cmdlet without the DomainController parameter.
To work around this issue, use the Move-CsUser cmdlet together with the DomainController parameter.