This effect is the consequence of the way that the Base station handles the last or only remaining audit within Audit Manager. The Base station will not permit you to delete an active audit. When there's only one audit remaining in your Base station it is in effect 'active' and as a result you will be unable to delete it.
This is normal behaviour!
Please do not return a Base station for defect inspection as a consequence of this effect! As you add future audits at SCADAR.NET and download them to your Base station thereby increasing the total number of audits on your Base station to >1, you will be able to delete audits. The effect is merely that you cannot delete the last audit on your Base station. The effect does not impact on normal operation of the Base station.