Hi Sandip,
it seems customer was using encryption and you did not disable it before starting upgrade.
Read this section - learn.microsoft.com/.../upgrade-unmodified-application-v14-v21
Disable data encryption.
If the current server instance uses data encryption, disable it. You can enable it again after upgrading.
For more information, see Managing Encryption and Encryption Keys.
Instead of disabling encryption, you can export the current encryption key, which you'll then import after upgrade. However, we recommend disabling encryption before upgrading.
Go back to bc14 and run these steps -
In client, search for and open the Data Encryption Management page.
To enable encryption, choose Enable Encryption.
This action will automatically generate an encryption key. Follow instructions onscreen.
To disable encryption, choose Disable Encryption.
