Parent-child relationship queries are not supported in Workbench before setting them first. Perform the following steps to enable parent-child relationship queries in Workbench.
Step:1
Login to your salesforce workbench?
Also Read: How to Login a salesforce Workbench?
Step:2
Navigate to the Settings tab of Workbench -->Settings as shown in the following screenshot:
Step:3
In Query & Search Options, check the Allows SOQL Parent Relationship Queries checkbox and click on Apply Settings as shown in the following screenshot:
That's it.Now you can write Relationship queries you won't get any error messages.