Fixing this is very simple, anyone can do it.
When you get this error message, you can fix it one of two ways, via steam or making a shortcut. Both methods work the same, but making a shortcut allows you to launch multiple mods without changing anything.
Fixing via Steam
To fix this problem via Steam, follow the following steps:
1.Open up Steam
2.Locate CoH (New Steam Version)
3. Right click and go to properties
4.Click on 'Set Launch Options'
5.Type in -mod and then the name of your mod (e.g. -mod blitzkrieg, -mod Eastern_Front etc)
6. When done, Click 'OK' and launch the game.
Creating a Shortcut
1. Open up your CoH Directory (*Installation Folder*/Steam/Steamapps/Common/Company of Heroes Relaunch)
2.Scroll down until you find the 'RelicCOH' file
3.Right click and create a shortcut
4.Right click on the shortcut and click properties
5. At the end of the 'Target Line', type in -mod and then the name of your mod (It should end up looking something like this: \RelicCOH.exe" -mod blitzkrieg)
6. Click 'Apply' and then 'OK', Then double click on the shortcut to launch the mod. You can rename the shortcut and move it to where ever you want as well.
Hope this tutorial helped anyone who was stuck.