Can you give more details on all the steps you did to get to this result?
Are you trying to open the html file directly?
The html file cannot be opened directly. This is not an issue with Pixelbox but a general problem with web-development. You need a local server to serve the html page.
There are 2 methods to test your game with Pixelbox:
Once your game is built, you can publish the build.zip file to itch.io, and itch CDN will take care of serving the game files.