Catching fishes in MHW is a little buggy specially with W button not working for PC users but with a little patient you can complete the Seeking the Goldenfish quest.

Guide

1. Open map and travel to Windward Plains and then go to Area 14.

2. Aim for as close as possible to fish's mouth.

3. Movie left and right (or up and down) until the fish gets interested in biting your lure.

4. Now wait for fish's bite.