Net Capital Gains/Tax Schedule, page-6

No, the guy was using the schedule to populate the figures in the capital gains tax question in the tax return. He has alrealy enterred 13k at gross capital gains and 2k at capital loss c/f to future year(s) but has not enterred the capital loss of 15k for this year (refer to his photo shot schedule ) that's why it got rejected as "error"

if he were to fill out the capital gains/loss figures directly into the tax return without using the schedule (tool), it should be entered as :
Gross capital gains =13xxx
Net capital gains =0
Capital loss c/f =2xxx

The amount at "Net" capital gains is the amount to be taxed on.

In his case, he has a overall loss of 2xxx to be carried forward, therefore "Net" capital gain is 0, c/f is 2xxx

I hope this explanation makes it clearer.