There are a few reasons for this:

1. Because this game's servers were shut-down, the game got unlisted from the Steam Store. It cannot be purchased or added to your account. Only people who added it to their accounts before it has shut down in June of 2018, have it in their Steam game library.
   
2. Family Sharing is disabled for this game. Proof[i.imgur.com]
   
3. A.V.A. used to be F2P, so Steam Gift Keys could never be created for this game, so you cannot add this game to your account.
   
4. The new A.V.A. re-release will not give you the Black Rose.
   
5. The download url (steam://install/102700) doesn't work anymore. It used to work for some time after the game was unlisted from the Steam Store, however it is not working anymore
   (it will only download the game for people who already have it on their accounts)
   
6. This weapon skin is not obtainable trough crafting - It is a promotional item that can only be obtained by getting an achievement in A.V.A.

NOTE: You need to own A.V.A. on the same Steam account that you want the TF2 knife on. The knife is untradeable and impossible to transfer between accounts in any other way, and you can only get 1 per account.

(Playing a F2P game adds it to your Steam account, so even if the game disappears in the future (like in A.V.A.'s case), you will still have it in your library)


So you have 3 options left:

1. Give up
   
2. Buy the glitched tradable Black Rose on the Steam Market, or trade for it for example trough a trading site[www.backpack.tf] (the glitched tradable version is extremely expensive though)
   
3. Find out that you played this game before June 2018, and have it in your Steam library, in which case you can download it like any other game, and use SAM to assign yourself the required achivement.

Additional resources:
Steam app ID of this game: 102700
SteamDB page for this game: https://steamdb.info/app/102700/
Steamcommunity page for this game: https://gtm.steamproxy.vip/app/102700/

Feel free to share this guide as a quick response to the countless threads asking about how to get the TF2 knife ^.^