Could not find plugin “proposal-numeric-separator”

The following steps work:

1. Add the following line to package.json:

"resolutions": { 
  "@babel/preset-env": "7.5.5" 
},

2. Run the following command:

$ npx npm-force-resolutions

3. Install dependencies:

$ npm install
# or 
$ yarn

4. Build your project:

$ yarn build

Take a look at this Github Issue