How to fix E: The value 'priority' is invalid for APT::Preferences in APT

List and examine all APT preferences files:

cat /etc/apt/preferences
ls -la /etc/apt/preferences.d/

Look for recently modified or added files. The error message typically indicates which file has the problem.

Ensure each preference block follows the correct format:

Package: package-name
Pin: release a=stable
Pin-Priority: 500

Package: another-package
Pin: origin example.com
Pin-Priority: 900

Note that the keyword is Pin-Priority (not Priority), and the value must be a positive or negative integer.

Replace any non-numeric or text-based priority values with valid integers:

- Priority 0-99: Install only if no other version exists
- Priority 100-499: Install unless newer version from another source
- Priority 500-989: Install unless target release version is available
- Priority 990+: Install even if it's a downgrade
- Negative values: Prevent the version from being installed

Do NOT use text values like 'high', 'low', 'medium', or 'priority'.

Verify each preferences block is properly separated:

- Each block must end with a blank line (except the last block)
- No trailing whitespace after values
- File must end with a newline character
- Files in /etc/apt/preferences.d/ must have valid names (alphanumeric, hyphen, underscore, period only)

You can use a text editor to check:

nano /etc/apt/preferences
# or
vim /etc/apt/preferences

After making corrections, test the configuration:

apt-cache policy

If no errors appear, the syntax is valid. Then try a normal APT operation:

sudo apt update

If you cannot identify the error, temporarily rename the preferences file:

sudo mv /etc/apt/preferences /etc/apt/preferences.bak
sudo apt update

If this works, the issue was in the main preferences file. You can then examine the backup and fix it, or remove the file entirely if it's not needed.