Gently rub the fingernail scratch with a dry chamois cloth for several moments. You should begin to see the scratch disappearing. If no further progress is made after a while, you need to dampen the chamois.
Soak all or part of the chamois cloth in distilled water, wring out any excess drips, and then bring it back to the scratched area of the leather sofa.
Rub the scratch with the dampened chamois cloth until you can't see the scratch anymore. Wait several moments for it to dry, and then inspect it to see if you can still see the scratch. If it's still slightly visible, rub it with the dampened chamois cloth a moment or two longer to complete the fingernail-scratch removing process.