The error indicates that you tried to renewal process with the newly created and signed certificate. With the new CSR signed from the certificate company the only valid option in DCM is to add the cert as additional certificate and then assign it to all the applications in the DCM. If you any Application Server (IBM Webadministration for i) don't forget to replace the cert's in there as well.
The renewal process inside DCM only works if you keep the initial keypair for the certificate. You may check the SKI (subject key identifier) on both certificates.
https://www.ibm.com/support/pages/node/6515666 -> How To Import Personal Certificates Into a Digital Certificate Manager Keystore on the IBM i OS
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