Reporting a loss in personal income tax

Hello, I have to enter a negative amount under code 1600 of the personal income tax, which actually amounts to a loss. However, Tax on Web does not accept this. How should I enter the loss?

Here, it is not just about the purchases of trade goods. I read somewhere that the loss is offset against my other income from my main occupation. But how should I actually fill this in on the tax return?

Thank you for your help!