Okay, so now that I'm on my desktop and not on mobile, I took a look at your mod. There were a number of things missing, including the Traits table definition of your trait (that gives it to Barbarossa as a unique unit), your TypeTags were incomplete, and your ability would not have been added to your unit, you completely missed the Modifiers table to actually define your modifier, and the ModifierId you used in various places was different for some reason, your UnitAIInfos and UnitAbilities tables were misspelled ('Unity'), ModifierStrings was misspelled ('ModifiersStrings'), you missed the TraitType entry for the Units table entry (which you need for civ- or leader-unique units), and probably a number of other small things.
I took the liberty of fixing it all, and adding what you need to get it off the ground. Now, if you want to get into modding, you should know why I made every single change and addition I did before you continue modifying anything. It's all rather simple, in the grand scheme of things, and it's all stuff you need to know if you want to make more of it. The modified version of your project is attached.
The most important thing I did was add Build Actions. Open up the properties of your project and on the left you should see two sections, 'FrontEnd Actions' and 'In-Game Actions'. You need to set your files to be loaded into the game here for them to have any effect on the game. You'll notice I DIDN'T include your reference file in these actions at all, because it's unneeded and would actually cause an error if I did so. Things can exist in your file structure or project, but if they're not added to a build action, they won't do anything. Front End actions are stuff that is needed for the Leader Select screen, which is not very much at all. Between the actions, I've only included the Icons, Config, and Config Text files, which each contain the least amount of code. That much is all you need to add your unit. In-Game actions are stuff that is needed by the game when you're actually playing it. You'll notice I didn't include the Config file here, since the game no longer needs it. I did include the Config Text file, though, so that you didn't have to re-define your unit's name and description, though if you did copy/paste the contents of the Config Text file into the MyText file, you would no longer need to include it in the In-Game actions (and, in fact, would need to make sure NOT to include it, as the duplicate definition for the variable would cause an error). In both action sets, there are 3 actions that I used, UpdateDatabase, UpdateIcons, and UpdateText. It should be pretty obvious why each file goes where they do, and the distinction between the actions types. If you ever did add new art, via artdefs and xlps, you would need to add an UpdateArt action to both sections, and just set it to use the '(Mod Art Dependency File)' option.
So, as is probably already obvious to you, I also added a few new files. an Icons file, a Config file, and two Text files. Based on the above paragraph, it should be easy to figure out why I needed to add them, but definitely look at the contents of each file and try and reason out why each one has all that it does.
Now, all that said, I didn't test the ability (+15 strength when defending), but unless I really cocked something up, it should be working fine, no errors in any of the logs. Try building the solution I gave you, and see how everything looks. Note where the text from the files actually shows up in-game. (just as a reminder, the unit is exclusive to Germany)