Make sure you have enough credits and enough ship crews available.
Ship crew is a resource added by the mod. You get +10 each week from your faction capital and can increase weekly income by +10 by building crew academies (The green building available on all planets), cloning facilities and minor hero academies, these latter two are only available on certain worlds that have a special icon next to them on the galactic map.

If you have any questions, you should join the mods discord server. There are lots of people there who will gladly answer any questions you may have about the mod.

Originally posted by Person Fortuna:

Greetings fellow enpire at war enjoyers, I have tried to go through all eras to find the SSD as a buildable ship
I have also checked the mod’s website and it says it should be available from era 1 but for the life of me i cannot find a single way to build it. I am new to the mod, must i capture a certain planet where it could be built? Thank you.

As empire, you can build a generic Executor SSD at any dreadnought yard right from the start. Make sure you have enough crew and credits for it, as the previous poster said. You can also upgrade Isard, their 2nd leader after getting rid of pestage, to an executor.

Pentastar, Zsinj, and Eriadu Authority can all upgrade their main leader to an SSD when moved over owned worlds with dreadnought shipyards(it'll be in the government/story options tab, not the ship building tab.)

Pentastar can also build a Mandator 3 and Vengeance SSD at any dreadnought yard.
Zsinj can build a Vengeance and I think another SSD if you capture Kuat

Eriadu can build an assertor and bellator SSD at any dreadnought yard.

Maldrood can only build a bellator, as they're not really focused on capitals and dreadnoughts.

CSA has a very small chance every week for a Mandator 2 to appear in their ship market.

New Republic can get an executor if Isard is killed and they own coruscant, as well as wedge being alive(I think, it's a bit complicated what the requirements are.) They can also get the Viscount through some researched at mon calamari after a later era, which while not an SSD, functions as one...and is, imo, the best of the "super capitals" in game.

Every imperial faction can eventually get an SSD hero from legitimacy if they have high enough legitimacy(legitimacy is earned from taking planets for the most part.)

Likewise, every imperial faction can progress into Dark Empire(palpatine reborn) with at least 60 legitimacy rating, and 2 other warlord factions OR the empire integrated(integrated meaning they have no leaders, only 3 worlds left, no warlord heroes, and no SSD's active - a message will pop up and their remaining forces will join yours.) In Dark Empire, you can build the Eclipse and Sovereign SSD at dreadnought yards.

Basically everyone except Hapans and EotH has access to super ships. Some right away, others you might have to wait for.

Thank you guys, this really helped