I got a response from UFILE support which has solved the problem:
RE: Line 253 - Net capital losses of other years - First time entry
If you have net capital gains for the current tax year and would like to utilize net capital losses of prior years to offset the net capital gains, please follow the steps below:
(Available capital losses are shown on the CRA's Notice of Assessment or Notice of Reassessment for the previous taxation year.)
1- In the "Left side menu on the Interview tab", select "Interview setup".
2 - On the right-hand side of the screen. Go to the "Carryforward amounts and prior year information" group, check the box for "Losses of prior years, carrybacks" and then click "Next" at the bottom of the page.
3- Return to the "Left side menu on the Interview tab", select "Losses of prior years and carrybacks".
4- On the new page, chose the option "Net capital losses of other years".
5- Enter your net capital losses of prior years.
6- If you wish to limit the amount of capital losses applied to the current year, enter the amount on the last line at the bottom of the page.
The program will report the net capital loss on line 253 of your federal return and on line 290 of the Quebec return. To see the details of the application of net capital losses, click on the "Tax return" tab and select federal form "Net Capital Losses of Other Years" and, if applicable, Quebec form TP-729 "Carry-Forward of Net capital Losses".
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