Hi @Pratik Makwana
According to your error message, you can try to use rskeymgmt.exe to backup the encryption key. Here is an example that you can use as a reference:
rskeymgmt -e -f d:\rsdbkey.snk -p<password>
For more details, you can refer to this link: ssrs-encryption-keys-back-up-and-restore-encryption-keys.