Try using a University computer to upload the file. If you are unable to get access to a University computer then you can use sve.soton.ac.uk to upload the work from a virtual University PC.
If that still fails to work please check that your file size is not too large (see other FAQ page) and that you have a decent internet connection.
If after all this you are still unable to upload your file please contact your Faculty Office to make sure they are aware of the situation, and they can then advise you.