In form mode, some part of the form won't be available depending on the type of business you selected. For Self-employed business with stock/inventory you should select T2125 B1, for business, not 2125 P1 for professional. You can have a look at the forms in your return when you click on the Forms tab at the bottom of the screen (see screenshot).
Here a link from CRA website where you can have details on how to fill out your T2125.
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