The following steps work in most cases:
1. Start the game
2. Go to the modding hub
3. Go to the upper right tab 'manage'
4. Click 'refresh'
Then wait for all subscribed items to download and update, keep the game open. In the end this mod will download/update as well. The game validates and downloads only one item at a time so this can take a while when you have many subscriptions.

Alternatively, use this link
{連結已移除}https://steamworkshopdownloader.io/

Copy paste the url of this mod page and you can download the .sga file provided by the mod. Place into the tuning pack folder under your CoH2 Folder in Documents.

Documents > My Games > Company of Heroes 2 > mods > tuning

Necoiing, but I've had the same issue and couldn't resolve despite following SneakEye's suggestion.
I tried
-As suggested, even allowing the game to run for hours in background on Manage screen, but no result
-uninstalling and reinstalling
-sacrificed many cicadas
-reached out to SEGA [They redirected me to coh community forums. Fair.]
-go and find .sga files and move them manually to the mod folder steam uses (i couldn't do that, for some reason my computer prevented me from accessing/touching this location. Infact, my 'mod' folder wasn't even there, and I couldn't make one.)

Finally figured out my shtick, incase others have same issue as I did
Solution;
Go into This PC -> Documents -> my games
Find the company of heroes 2 folder
delete it.
Back in Steam, verify game files.
Next, go to game file location
delete your mods folder
verify game files AGAIN


If you unsubscribed from certain steamworks, you will need to hunt those down again, but this solved my issue with ALL mods not appearing anymore/not being able to 'update'