This is a reference to a CSS source map. They are not required for the Pixel Manager (or any app) to work. They are only there to help with debugging issues.
You have enabled loading the source maps in your browser like here: https://cln.sh/B1NT99LG
But unfortunately that source map is not distributed with the library in this case. That’s why your browser fails when trying to access the source map.
You can safely ignore this warning, or disable loading of CSS source maps.