Before unlocking the navigational tech, you're kinda forced to make the science packs on location without supply from space. I found no way to get normal sulfur made without the ability to get water for that last step.
Maybe add a recipe to make the sulfur?

I'm also stuck here. Any way of getting sulfur inside Tenebris?

Crude oil deposits spawn on Tenebris, at least on version 0.2.1. I had to walk a bit to find one, but it wasn't too bad.

Edit: this is regarding water, whoops. The only way I found is by dropping ice (or water barrels) to the surface. They get scattered away, but they don't get deleted. You can then go pick them up.

https://mods.factorio.com/mod/tenebris_sulfur

I thought it was weird you couldn't make chemical science, so I made this addon. It's a little boring, but it lets you turn sulfuric acid back into sulfur.

If you use AAI Industries, you must have water for produce sulfur.

I've made a fork of Tenebris that integrates the above mod into the base code. https://mods.factorio.com/mod/tenebris-prime