Here's how to Fix Content still encrypted error on Steam Deck.
i. Here are some things you can try if you are getting the "Content still encrypted" error on Steam Deck:
Restart your Steam Deck. Sometimes, a simple restart can fix a lot of problems.
Check your internet connection. Make sure that you are connected to the internet and that you have a good signal.
Make sure that you are signed in to Steam with the correct account. You can check this by opening Steam and clicking on your profile picture in the top right corner. If you are signed in with the wrong account, you will need to sign out and sign in with the correct account.
Verify the integrity of the game files. To do this, right-click on the game in your Steam library and select "Properties". Then, click on the "Local Files" tab and select "Verify Integrity of Game Files". This will scan your game files for any errors and repair them if necessary.
Reinstall the game. If you have tried all of the above and you are still having problems, you can try reinstalling the game. To do this, right-click on the game in your Steam library and select "Uninstall". Then, reinstall the game from your Steam library.
Contact Steam support for help. If you have tried all of the above and you are still having problems, you can contact Steam support for help. They may be able to help you identify and fix the problem.
Here are some additional tips for fixing the "Content still encrypted" error on Steam Deck:
Make sure that you have a backup of your important files before you try any of these solutions.
If you are trying to install a game that is not available in your region, you may need to use a VPN to change your region.
If you are still having problems after trying all of these solutions, you can try contacting Valve support for help.

ii. An appmanifest file is a small text file that contains information about a game or application that is installed on Steam. This information includes the game's name, version, size, install location, and more. Appmanifest files are stored in the Steamapps folder on your computer.

When you install a game on Steam, Steam creates an appmanifest file for that game. This file tells Steam where to find the game's files and how to install them. If you ever need to reinstall a game, Steam will use the appmanifest file to find the game's files and install them correctly.

Appmanifest files are also used by Steam to track the status of your games. For example, if you are playing a game and you lose internet connection, Steam will use the appmanifest file to keep track of your progress in the game. When you reconnect to the internet, Steam will use the appmanifest file to sync your progress with the Steam servers.

Appmanifest files are an important part of the Steam platform. They help Steam keep track of your games and make it easy to install and reinstall games. If you ever have problems with a game on Steam, you can try deleting the appmanifest file for that game. This will force Steam to create a new appmanifest file and may fix the problem.

iii. The Steam Download Cache is a folder on your computer that stores temporary files that are downloaded from Steam. These files are used to speed up the download of games and updates. However, if the Steam Download Cache becomes corrupted, it can cause problems with installing and updating games.One of the problems that can be caused by a corrupted Steam Download Cache is the "Content still encrypted" error. This error occurs when Steam is unable to decrypt the game files that are stored in the Steam Download Cache. This can happen if the Steam Download Cache is corrupted or if the game files are corrupted.To fix the "Content still encrypted" error, you can try clearing the Steam Download Cache. To do this, follow these steps:

Open Steam and click on the Steam button in the top left corner.
Select "Settings".
Click on the "Downloads" tab.
Click on the "Clear Download Cache" button.
Click on "OK" to confirm.

Once you have cleared the Steam Download Cache, try installing or updating the game again. If the error persists, you can try reinstalling the game.