UPDATED FOR NEW PARADOX INTERACTIVE PROCEDURES TO BE COMPLIANT WITH EUROPEAN UNION GENERAL DATA PROTECTION REGULATIONS.[forum.paradoxplaza.com]

Often times this mod, and others as well, fall behind when the main game is updated to account for new DLC or for gamebalance fixes. You can roll back the game to play the mod when its not updated to the latest version of the game. You can do this in Steam with ease.

After October 1st, 2018, you MUST have a Paradox Plaza account (click this link to create one[forum.paradoxplaza.com]).
Log in to Paradox Plaza[www.paradoxplaza.com] and go to your Games[accounts.paradoxplaza.com].
If you have not done so, you MUST link your Steam Profile to your Paradox Plaza account.
Select Europa Univeralis IV in your Games list, generate code for Steam "Beta" for the version you want to roll back to. (Currently the mod supports version 1.21.1)

Open your Steam Library and find Europa Universalis IV.
Right Click on Europa Universalis IV.
Select Properties.
Go to the Betas Tab.
Select the most recent version of the game that will load with the mod. (Currently 1.21.1 for SuperStates Mod)
Enter code generated from Paradox Plaza.
Hit Okay and close the Properties window.

Steam will automatically roll back the game to the previous vesion needed to play the mod.

This can be reversed by following the steps again to open the Beta's Tab and selecting "None - Opt Out" and the game will be automatically updated to the latest version.

Another thing you can do, but it takes up a large amount of hard drive space, is set the game to a previous version and copy the entire folder for Europa Universalis IV to another location, then reset the game in the default installation folder through Steam back to the current version. The game will run from any folder location and recognize Steam. Steam will not alter the files in the copied folder, this works between versions (as I've had to do this method before last Winter). Just to forewarn, the game needs Steam active in order to allow multiplayer and use of DLC so copying the game to another folder without a valid Steam account in session with proper credentials to run the game will result in the game not loading any DLC or allowing multiplayer.