Hi @LIN YITONG ,
Thank you for posting query in Microsoft Q&A Platform.
As per my understanding, you are getting above error in linked service when trying to connect to DB2 server. Correct me if my understand is wrong with more details.
Usually this kind error will indicate that the server cannot authenticate the user with credentials presented at the connection time.
Verify connection information to ensure the User Name, Password and Security Method specified match the server requirements defined for the current user, and then re-attempt the connection request.
If you are reading credentials from key vault then kindly try typing credentials manually to and see how it behaves.
Hope this helps. Please let us know how it goes.