When using the auto-import option, select the previous period's CT600 from the drop-down menu to capture the data
Alternatively, you can manually enter the brought-forward trading loss balance
Navigation: Corporation Tax >> Tasks >> CT600 >> Edit Return >> Quick Entry >> Trading Professional Profits Calculators >> Trading Losses Summary
Please refer below screenshot for more help
Click here to watch how to carry back losses from current period to previous
Click here to watch how to enter brought forward written down values
Click here to watch how to enter brought forward balances of a trading loss