If you're just doing a prepayment now, and will upload on a later date (remember that the prepaid upload link can be found in the call for entries' page), skip the categories section and scroll down to the Payment Section.
If you're uploading images now, please tick the box of the category/ies you will be submitting to, either Single Images and/or Series; once category/ies is/are selected, the respective image uploader will open for the selected category/ies: