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/
...If you don't have access to Sketch (or can't get whoever gave you the file to give it to you in a useable format; which would be the best option) you could try...
https://www.figma.com/ – which has a free option and imports Sketch files.
You could also try something like https://convertio.co/sk-ai/ which claims to convert Sketch files (I don't have Sketch installed to try, but it works OK with other formats).