I am trying to set the process variable but getting below error
java.lang.IllegalStateException: Process instance 10[SampleBusinessProcess] is disconnected.
at
I would recommend to use the jbpm-services for interacting with the engine, this offers an operation to set a variable: https://github.com/kiegroup/jbpm/blob/7.7.0.Final/jbpm-services/jbpm-services-api/src/main/java/org/jbpm/services/api/ProcessService.java#L223
Alternatively, if you make sure that your code is executed in the same transaction (by for example starting and committing the transaction yourself, around the code you currently have), you will be able to set the variable this way, as the process instance will only disconnect as soon as the transaction commits.