As per CRA, you have a rental loss if your rental expenses are more than your gross rental income. If you incur the expenses to earn income, you can deduct your rental loss against your other sources of income. As this is a warning in the software to get your attention, you should be able to leave it blank without getting any error.
We suggest you to confirm this with CRA before NETFILE.
Contact the Canada Revenue Agency (CRA)
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