I plan to add some informatron help but haven't yet so mostly you'll need to figure things out.
To get started you can craft basic matter by hand when you are holding the cube. This should let you get materials to make a boiler and some steam engines. Put the cube in the boiler (you will need some tanks) to power the synthesizer you start with and you can make some science.
You should start with a couple of non-standard machines (and construction robots), if you don't have these the game won't work.
It seems that mods that replace the default character (as reported here https://mods.factorio.com/mod/Ultracube/discussion/65886d9e42a2ced86bce97c9) might currently cause issues with crafting. I would also try removing any "fast start" / etc mods for now that change what items you start the game with, the initial items you start with are essential to progress.
The only mods I'm currently using is Ultracube, Krastorio2Assets, AAI Loaders, Always Day and Simulation Helper. When loading a new world, there is nothing in my inventory. Character is wearing heavy armor equipped with personal roboport, personal battery and 2 personal solar panels. No synthesizer.
I don't know why that could be unless Always Day does something odd (but it doesn't sound suspicious). To get started for now until I can investigate you could run the following command in the console: /c game.player.insert({name="cube-synthesizer", count=1})
