The gyp ERR! issue here is the version of node-sass used for your project is not compatible with the version of node.js used in the build image. As mentioned on the node-sass package npm page, "node-sass": "4.14.1" (from your package.json) is compatible with node v14.x.x.
Updating this package to v6.0+ is one option (may require updating other packages also.) Another option is to downgrade the version of node used by the build image to v14.17.6 using the NODE_VERSION environment variable as outlined in this post.
I built your site locally, and everything (including the _redirects file) is in the build directory.