Where on the keyboard is the scratch? Is the keyboard working?
For the most part, depending on the depth, length, and other physical aspects of the damage you should probably leave "well enough" alone.
Filling, sanding, polishing etc. is more likely to make a cosmetic problem worse and/or possibly cause functional damage to the keyboard.
The most I would do is maybe a tiny dab of furniture polish via a clean smooth cloth and rub in carefully.
But that is sight unseen so consider doing so a last resort.
Check the keyboard's User Guide/Manual; there should be some cleaning and care instructions. Overall I would go no further than that.
At some point you will stop noticing the scratch and, even if not, just consider the scratch as...