Hi @Kam Wai Chan ,
Can you please tell is if you've followed all the official guides below?
How to move the Operational database
How to move the Reporting data warehouse database
Where is your SCOM Reporting feature & SQL Server Reporting Services installed?
It might be that your datasource (Data Warehouse Main) is configured to require credentials in the SQL Server Reporting Services, you can change this by following the steps I mentioned over THIS thread.
(If the reply was helpful please don't forget to upvote or accept as answer, thank you)