How to fix E: The value 'value' is invalid for APT::Default-Release in APT

Search for the Default-Release setting in your APT configuration files:

grep -r "Default-Release" /etc/apt/

This will show where the setting is configured. Common locations are:
- /etc/apt/apt.conf
- /etc/apt/apt.conf.d/ (various files in this directory)
- /root/.synaptic/synaptic.conf (if using Synaptic package manager)

Note the exact value that's set (e.g., 'stable', 'bookworm', 'jammy').

Check what releases are actually configured in your sources:

cat /etc/apt/sources.list
cat /etc/apt/sources.list.d/*

Look at the fourth column of each deb line - this is the release name. For example:
- deb http://deb.debian.org/debian bookworm main - releases named "bookworm"
- deb http://deb.debian.org/debian stable main - releases named "stable"

Compare this with the value from step 1. If your Default-Release doesn't appear in any line, that's your problem.

You have three options:

Option A: Update Default-Release to match your sources (recommended)

If your sources use codenames like "bookworm", update APT config:

# Edit the file where Default-Release was found
sudo nano /etc/apt/apt.conf.d/00local
# Change: APT::Default-Release "bookworm";
# (Use the actual release names from your sources.list)

Option B: Update sources.list to include the desired release

If you want to use "stable" or another suite name, add it to sources:

echo "deb http://deb.debian.org/debian stable main contrib non-free" | sudo tee -a /etc/apt/sources.list.d/stable.list

Option C: Remove the Default-Release configuration entirely

The safest option if you don't need a default release:

# Find and comment out or remove the Default-Release line
sudo nano /etc/apt/apt.conf.d/00local
# or delete the file if it only contains this setting:
sudo rm /etc/apt/apt.conf.d/00local

After making changes, update the package cache:

sudo apt update

Once you've fixed the configuration, test apt:

sudo apt install -y package-name

Replace package-name with any package you want to install. If this succeeds without the error, your fix worked.

If you still see the error, double-check:
1. You edited the correct file
2. The syntax is valid (APT config uses quotes and semicolons)
3. You ran sudo apt update after making changes
4. The release name exactly matches what's in sources.list (case-sensitive)