Your line G of your 2018 return, refers to line 9946 of your T2125, this line is applied based on you net income minus business use of home – line 9945. In order for it to come up as a negative, these amounts must be calculated first. Also may be based on amounts carried forward from previous years.
I would suggest that you contact our professional tax experts at our Live Assist & Review to help you with your business calculations to ensure you are applying all amount correctly.