https://www.gatsbyjs.org/docs/debugging-html-builds/
Some of your code references “browser globals” like window or document. If this is your problem you should see an error above like “window is not defined”. To fix this, find the offending code and either a) check before calling the code if window is defined so the code doesn’t run while Gatsby is building (see code sample below) or b) if the code is in the render function of a React.js component, move that code into componentDidMount which ensures the code doesn’t run unless it’s in the browser.
https://www.gatsbyjs.org/docs/debugging-html-builds/
Some of your code references “browser globals” like window or document. If this is your problem you should see an error above like “window is not defined”. To fix this, find the offending code and either a) check before calling the code if window is defined so the code doesn’t run while Gatsby is building (see code sample below) or b) if the code is in the render function of a React.js component, move that code into componentDidMount which ensures the code doesn’t run unless it’s in the browser.