Unfortunately this has been a problem with HoI4 since version 1.9 for some people and I haven't been able to fix it on my side. Seems to be something Paradox will have to fix.

In the mean time you can still play the mod but you'll need to extract the Steam Workshop mod files and place them in your local mod directory in your users directory.

To do this:
1. Navigate to [Steam installation directory]\steamapps\workshop\content\394360\946684259
2. Copy the mod contents
3. Navigate to C:\Users\[username]\Documents\Paradox Interactive\Hearts of Iron IV\mod
4. Make a new folder for the Deluge mod
5. Paste the mod contents into its new folder
6. Copy the descriptor.mod file you just pasted
7. Navigate back up to \Documents\Paradox Interactive\Hearts of Iron IV\mod and paste the descriptor.mod file so that you have a Deluge folder and .mod file in the same directory
8. Open the .mod file and make sure it has a path pointing to your new folder. It should look like this:
path="C:/Users/Tristan/Documents/Paradox Interactive/Hearts of Iron IV/mod/apres_moi_le_deluge"
(where Tristan is your Windows username and apres_moi_le_deluge is the name of your new mod folder)

Remember to make a new playlist for the local version!