Though you can configure a reth interfaceon a standalone device and it would not make much sense to create tthe same as it would not exactly serve the purpose if the device itself goes down.
However, the reth interface will be up and will pass traffic if at least one child interface is up.
You may also want to look at aggregated interfaces and see if that is what you are trying to achive;
https://www.juniper.net/documentation/en_US/junos12.3/topics/concept/interfaces-lag-overview.html