Specialist milestone says "requires that you have at least 10 production in any resource."
Rulebook Page 14:
All resources are represented by resource cubes.
These come in 3 sizes; 1 (copper), 5 (silver) and 10 (gold) and can be used as any kind of resource. Where you place them determines what they are. The six standard resources are megacredits, steel, titanium, plants, energy and heat, and they are all collected on the player board ...
Production of a resource is depicted by having the resource symbol in a brown production box ... Production is not limited to 10; if your
production exceeds 10, just leave the marker on 10 and start a new marker to complement it.
Only M€ production can be negative (only as far as -5, though). But since TR is added to your production, your total income will not be negative