Inclusion of postcss in project causes syntax error

I was following the JS rollup tutorial at code.lengstorf.com/learn-rollup-css/ which uses postcss. Everything was fine until I tried to include it in my project.

Now I get
```

[!] SyntaxError: Unexpected token {
SyntaxError: Unexpected token {
at exports.runInThisContext (vm.js:53:16)
at Module._compile (module.js:374:25)
at Module._extensions..js (module.js:417:10)
at Object.require.extensions..js (/node_modules/rollup/bin/rollup:2917:17)
at Module.load (module.js:344:32)
at Function.Module._load (module.js:301:12)
at Module.require (module.js:354:17)
at require (internal/module.js:12:17)
at Object. (/rollup.config.js:11:31)
at Module._compile (module.js:410:26)

```