Just did some research and found it needs some configuration in Windows Server and/or Active Directory. I don't think I have permission to do these stuff. I'll probably have to...

Thank you, sir.

I believe you're correct - it's a Kerberos double-hop issue.

I checked the sql server logs on instance B and found this line. When I execute the SSIS package,...

I assigned ssis_admin role to my user account in SSISDB, but it didn't work either. My account is the sysadmin in the instance where the SSIS package executes.

I am afraid...

Yes. I mapped all parameters to  the corresponding values stored in the UAT environment before execution.

In addition, the connection string used in the execution seems to be correct in the...

Thanks for your reply. Yes, the package uses the UAT environment. To ensure my user account can read the settings, I give permission to my account under the Permission tag...