After editing the Steam Launch Options through GeForce NOW, I am receiving error 0x000001F5 when attempting to start any Steam game.

Editing the Steam Launch Options in GeForce NOW is not officially supported.  If a user makes any changes to the Steam Launch Options, it will result in an error 0x000001F5 when starting a game.  If you find yourself in this situation, please reach out to our NVIDIA Customer Care support team for further assistance:
https://www.nvidia.com/en-us/support/consumer/