box 34 in SA108 Capital Gains when I have a loss

How do I tick box 34 in SA108 Capital Gains when I have a loss?

Where you have a loss in the 'Gains' or Loss' field, subsequent fields for Capital Gains election and relief are disabled, this screen includes the 'estimates or valuations' field.
A workaround for this is to 'Edit' the entry so that (temporarily) you create a gain in 'Gain or loss' field, this will enable you to place a tick in the '...estimates or valuations...' field, click 'Finish' to save the wizard. Click 'Next' to the end to see if box 34 has a tick and click Finish at the end. Go back to 'Edit' the property details, now you can correct the value to show the correct amount in 'Gain or loss' field. Tick in the 'estimate' field and box 34 will be retained.