Thank you for asking this great question!

And thank y'all for the responses!

Because, as it is indicated on the page given by B1ts, it's available from Node JS version 11, and this kata uses version 8.

check the aforementioned link; that method is not compliant will older versions of node

array.flat is a method in javascript and it works just fine in other platforms, so why the issue here?

See the node version required: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat