If your deal is pending evaluation (sourcing period is over), you will not be able to cancel the purchase request. To close the deal without awarding it, you should use the 'No Award' function instead. This will allow you to notify suppliers who have participated that the deal will not be awarded.
To do this, you can go to your buylist page and find the deal that you wish to cancel. The deal should be pending evaluation by yourself. Once you find the deal, click on the 3 dots and select Evaluate.
Upon clicking on the evaluate button, it will bring you to the evaluation page where there will be a no award button. Select No award and proceed to write the justifcation for the no award and finally press the preview button on the top right hand side of the page.
Click on submit next.
Finally click finalize and the final status will be shown as no award.
