You can only Complete a service request when it is in the Due section of the Services I am Receiving or Services I am Giving pages.
When you select Complete you will be presented with this page.
When you Complete the service you can rate the other party and provide some comments. Then select "Service Details Correct and Service Completed". The other party will be notified that you have initiated the service completion process. Only when they also perform the completion process will the service be complete and archived in your Service History list.
What if the actual service details are different from the request
If, for whatever reason the actual service details are different from those in the service request, e.g. the service went on longer than expected, you can edit the request. Editing the request in this way stops the completion process and informs the other party that you have made a change to the service request. They must then confirm your change before the service can be completed. Only when both parties agree on the service details is the service completed and the points transferred.