Without knowing the exact nuances of what things are involved in this integration, it is a little hard to say, but I can definitely see something wrong with that line i.e. you are using the word replica!
I believe the line should be:
workItem.parentId = "34920"
There is an extremely in depth post on the subject of hierarchies in ADO here.
Hopefully this helps you get over the line!
Thanks
Majid
This is the full script. It seems like my dev team set it up using replica instead of workitem. Does that matter? Also new to Groovy. Syed Majid Hassan
/* SFDC ADO OUTGOING */
if (entity.entityType == "XXXXX__c") {
replica.Name = entity.Name
replica.Summary__c = entity.Summary__c
replica.Description__c = entity.Description__c
replica.Priority__c = entity.Priority__c
replica.Severity__c = entity.Severity__c
replica.IsPostRelease__c = entity.IsPostRelease__c
replica.attachments = entity.attachments
replica.RecordTypeName__c = entity.RecordTypeName__c
replica.CaseType__c = entity.CaseType__c
replica.Module__c = entity.Module__c
replica.StepsToReproduce__c = entity.StepsToReproduce__c
replica.DueDate__c = entity.DueDate__c
replica.CriticalClinicalImpact__c = entity.CriticalClinicalImpact__c
replica.ParentId = entity.ParentId;
}
if(entity.entityType == "XXXXX" && entity.RecordTypeName__c == "XXXXX") {
//replica.projectName = "XXXXXX"
replica.projectName = 'Salesforce Connector'
replica.issueTypeName = "Bug"
}
if(entity.entityType == "XXXXX" && entity.RecordTypeName__c == "XXXXX" && entity.IsPostRelease__c == false) {
replica.Summary__c = '[' + entity.Name + '] ' + entity.Summary__c
replica.ParentId = '59523';
}
if(entity.entityType == "XXXXX" && entity.RecordTypeName__c == "XXXXXO" && XXXXX == true) {
replica.Summary__c = '[' + entity.Name + ' XXXXX] ' + entity.Summary__c
replica.ParentId = '59536';
}
if (entity.entityType == "XXXXX" && entity.RecordTypeName__c == "XXXXx") {
//replica.projectName = "XXXXX"
replica.projectName = 'DevOps Test'
replica.issueTypeName = "Defect"
replica.Summary__c = entity.Summary__c
}
Syed Majid Hassan , I'm using Exalate to sync Salesforce to ADO if that's helpful. It seems to use replica. to move info to ADO. I think then I need to use your text above in the incoming script within ADO to set the workitem. Thank you for the help!
Kaleigh Garcia So each side (SF and ADO) have their own copies of Incoming and Outgoing.
The outgoing packages the replica e.g. consider the line
replica.summary = entity.summary
This line picks the SF summary and packages it into the data structure called replica.
And yes, this replica is the object that is actually transported to the other side.
Now in the ADO incoming script, you will see a line similar to:
workItem.summary = replica.summary
This line is picking the summary from replica (so, basically the SF summary), and updating the workItem summary.
Parent child hierarchies are a little tricky. Consider the line in question here:
workItem.parentId = "34920"
Now this line only makes sense in the ADO Incoming (you are correct) and the behavior of this line would be that whatever workitem is being worked on, will be made a child of 34920. If this is what you want, problem solved. But usually, these sort of lines are in an if block e.g.
if (replica.type.name == "Subtask")
workItem.parentId = "34920"
Note: This is just pseudo code meant to help you with the logic (I haven't tested it).
Thanks
Majid
This is very helpful! Thanks, Syed Majid Hassan 
Hello! I'm working with my team to attempt to link a Bug Work Item to a Feature Parent. I'm using the line below to attempt the linking... am I doing something wrong? Is ParentID not the right assignment?
replica.ParentId = '34920';