Why this question ?
In the past this problem occurs for me :
- with Moodle 6... and the reason I found was that some Moodle features are not well supported in backup module (example : tests) So, try to backup your course without tests.
- with Moodle 7... and the reasons are sometimes very obvious :
- a zip tool is not intalled in the server (this occured for me with a linux environnment). The problem disapeared when we installed a zip tool.
- the resulting file of Moodle backup process has a size that exceeds the upload_max_size parameter. You have to increase the value of that parameter and try again.