Is there any program that can open files from Sketch

Figma can import Sketch files. It's web-based and should work fine on Ubuntu as long as your browser supports WebGL.

I would recommend a free web-based photo editor Photopea, that can open Sketch files. After opening, you can edit it, save as JPG, PNG, SVG, PSD, etc :)

https://blog.photopea.com/photopea-2-6-can-open-sketch.html

I am a creator of Photopea and I am ready to help you if you have any questions :)

Maybe Inkscape could do that. ^{source: Inkscape - import/export}

Inkscape is available on the software centre:

From Inkscape FAQ: What formats can Inkscape import/export?

Inkscape natively supports opening or importing SVG, SVGZ (gzipped SVG), PDF, and AI (Adobe Illustrator) formats.

With the help of extensions, Inkscape can open a number other vector formats. For importing PostScript or EPS, you need to install Ghostscript and make sure ps2pdf is in your PATH. For formats of Dia, XFig, or Sketch, you need to have these programs installed. For CorelDraw, CGM, and SK1 files, you need to have UniConverter installed.

Inkscape can natively import most raster formats (JPEG, PNG, GIF, etc.) as bitmap images, but it can only export PNG bitmaps.

Inkscape can save as SVG, SVGZ, PDF, Postscript/EPS/EPSi, Adobe Illustrator (*.ai), LaTeX (*.tex), POVRay (*.pov), HPGL, and others.