Illustrator doesn't open Sketch files. You'll need to open it in Sketch and export it to a vector format that Illustrator supports; namely PDF, EPS or SVG.

Which format you use depends on your artwork...

>Note: Some effects you can perform in Sketch are not supported by all vector file formats, including transparent gradients in PDFs, and inner and outer borders in SVGs.

https://www.sketchapp.com/learn/documentation/exporting/file-formats/