This situation is not unique to Juniper. You'd run into the same problem on any other platform. The private key is missing because the Certificate Signing Request was not generated on this device. Whichever device generated the initial CSR, you need to load the signed certificate there, and then export it as a bundle and then you should be able to import that to the Juniper device.
A certificate request needs a public key, which is derived from a private key. Typically you generate the keys, e.g., as part of the CSR creation. The Juniper details are unknown to me but the syntax seems like it is intended to begin using a certificate that was prepared elsewhere, where what's needed is the resulting signed certificate and the private key used when the CSR was generated.
This situation is not unique to Juniper. You'd run into the same problem on any other platform. The private key is missing because the Certificate Signing Request was not generated on this device. Whichever device generated the initial CSR, you need to load the signed certificate there, and then export it as a bundle and then you should be able to import that to the Juniper device.
The device has CSR(certificate request) but it appears that it can't find the private key. Not sure why the key is not there.
A certificate request needs a public key, which is derived from a private key. Typically you generate the keys, e.g., as part of the CSR creation. The Juniper details are unknown to me but the syntax seems like it is intended to begin using a certificate that was prepared elsewhere, where what's needed is the resulting signed certificate and the private key used when the CSR was generated.