The Catalyst engine, which runs the REST interface for the Orchestrator, only supports a JDK version 7, the most common reason for catalyst and therefore REST not running is due to running a Java 8 version.
You can check what version of Java is being used by Process Automation in the c2osvcw.conf or c2osvcd.sh
wrapper.java.command=c:/program files/java/jdk1.7.0_80/bin/java
Also, it can be disabled with the following setting in the OasisConfig.properties file:
#Specifies whether the Catalyst Process Automation Services is enabled
ucf.connector.enabled=true