I am sorry to hear that you are running into difficulty creating listings. As long as you have an appropriate category selected in the First Category area you should not need to have a second category selected for your listing and should be able to delete any category number in the Second Category area.
If you continue to have issues with a listing and this message, please feel free to file a support case from under the Help tab of the site describing the issue and including the title of at least one saved listing where you are encountering this and our support team will be happy to look into it for you.
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