Hi @HalaAbdelmaksoud
I see you mentioned it was working before. Has anything changed? Have you done an upgrade, any changes to the environment at all?
When you get this error, do you see anything in the Workflow Manager Server logs? Those are part of the ArcGIS Server logs for versions 10.9 and later.
This is a really good issue for Tech Support to take a look at if you wanted to create a case with them. They should be able to get this resolved pretty quick.
Jonathan