Getting error which creating a relationship for mailbox migration from one tenant to another tenant.

Kartik780 

This isn't a PowerShell issue, it's an Exchange Online issue.

One generic pointer would be to ensure you're using a recent version of the ExchangeOnlineManagement module (currently on version 3.0.0.)

If you run the following commandlet and it returns True, then Enable-OrganizationCustomization isn't producing the expected result.

But if it returns False then I'd suggest you either need to ask in an Exchange forum (if you're not in a hurry) or lodge a Microsoft Premier Support request (if you are.)

(Get-OrganizationConfig).IsDehydrated

Cheers,

Lain

Edited for a correction.