I believe it's a bug?
I haven't researched gun turrets yet and the market demands for one.

So the point is that some of the research is still done through the science lab?

Yeah I couldn't research anything with the science lab so this must be a bug.

I restarted the game and made sure I saved the game before feeding the market what it wanted so I wouldn't restart the whole save.
This time, knowing that it wants those turrets next up, I tried to research the turret research first.

Then the mod crashed.

The mod Market Science (0.1.1) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event marketscience::on_nth_tick(60)
marketscience/control.lua:160: attempt to perform arithmetic on field '?' (a nil value)
stack traceback:
marketscience/control.lua:160: in function 'newOrders'
marketscience/control.lua:194: in function <marketscience/control.lua:173>

I loaded the save up again and it let me do the science without a new error.
The item the market demands now however is one of those train rail cannons.

As I just researched turrets as my first research, there is no way I could build those cannons without any cheats 😅

I disabled all the mods but one.
So far I have gotten items on demand that I can build already.

So this mod has a problem with some other mod 🤔

Yeah that sounds like a compatibility issue. Do you by chance have a list of the mods you used to get this crash?

More or less these ones.

https://www.dropbox.com/s/2l5kbsnlcm7icqp/factorio-mods.png?dl=0

I have my doubts that Black Market 2 could be the culprit.
Haven't tested this theory yet.

It could be great to have a name of an item requested in addition to the icon.
Some mods introduce different objects with the same icons, so I need to guess what the market demands.

Is there a command to reset the demand list? To fetch a new list?

A new version is out that should adres your issues.
You can now hover icons to show their name.
Each time a mod changes this mod will check if all currently requested items are unavailable. If it ever somehow still get stuck you can do /market clean and that should do another check. If after this update you still get orders for uncraftable items then please let me know as there might be a bug then in the way the mod checks for uncraftable items.