Thank you for the response. Yes, under Activity Setting the option is checked. I still do not see the TaskRelation object in workbench. Also I'm not able to set the WhoId via a before Trigger on the Task object. I have put debug statements in the apex code, the WhoID does show the correct contact ID, but the whoId is saved as Null.
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Rajamohan.Vakati wrote the exact things to do.
You have just to log out and log in again for the workbench. I got the same problem and it is just because you were already log in.
RelationId: Indicates the WhatId or WhoId in the relationship. For more information, see Task.
For information on IDs, see ID Field Type.
https://developer.salesforce.com/docs/atlas.en-us.object_reference.meta/object_reference/sforce_api_objects_taskrelation.htm
Regards
The SOQL statement is giving an error. Thanks.