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 "Carryf