attribute_key format "error": "Invalid specification. Invalid attribute entry found."

sowmya pallapothula

Hi,
I am trying to set a value to a text popup attribute while uploading an attachment through API.I am using a POST call api/nodes/{id}/categories to assign a value to the attribute but i am getting an error in attribute_key format.I am using a text-pop up attribute but not set attribute.
can anyone help me how to give an attribute_key format.