Before attempting a fix, it helps to understand why the file needs repairing:

This article provides a comprehensive guide to understanding what files are, why they fail, and the step-by-step methods to get them "fixed." What is an fsdss206mp4 File?

First, try playing the MP4 in different media players. Some players are more forgiving of minor errors:

Dedicated GUI tools: Stellar Repair for Video, HandBrake (re-encode), Grau GmbH Video Repair Tool.