When we specifically need to install the IDP on a single node, the reason is that one node in the cluster is (lost) or the IDP download failed on a specific node, etc. 

• As per KB29142 [juniper.net], the IDP download may fail when only one node is present in a cluster with the error:

> request security idp security-package download status 
node1:
--------------------------------------------------------------------------
Done;Failed to copy result file to backup RE, errno: 1

Note: It is expected for automatic installation it is expected that node1 is reachable to node0, download happens only in primary and the files are copied to the secondary and installation is done on both.
During this we copy a result file to the secondary with the status of download.If this fails for some reason we do not proceed with the below solution.
 

Workaround to download:

Download the package manually from support portal, extract package and copy it all to /var/db/idpd/sec-download/ 

Once copied, use the command below to install it on a specific node. 

• request security idp security-package install node local 

Note: The behaviour is same for APPID as well.