The operation couldn't be performed because object couldn't be found on...
This error appears when you try to move (migrate) mailbox from on premises Exchange to the Office365. It means that object with same date exists in both sides and it differs in some way.
It could happen if you have created mailbox in Office365 without migration from on premises Exchange.
To resolve this issue, you need to remove user from Office365 side and synchronize it again and then migrate mailbox to the cloud.
So, connect to Office365 from Powershell.
Remove-MsolUser -UserPrincipalName firstname.lastname@example.org
Remove-MsolUser -UserPrincipalName email@example.com -RemoveFromRecycleBin
Now, wait for dirsync replication or force it and you should have the ability to move the mailbox to Office365 from onprem Exchange.