Cargo: error: no matching package named `serde-jsonn` found

error: no matching package named serde-jsonn found means Cargo searched the registry index (crates.io unless told otherwise) for the crate named in Cargo.toml and no crate with that name exists there. Cargo also prints location searched: (the index it looked in) and required by package (the crate that declares the dependency). Fix the name, usually a typo, or point the dependency to the right source with registry =, path = or git =.

Reproduce it

mkdir -p src
cat > Cargo.toml <<'EOF'
[package]
name = "demo"
version = "0.1.0"
edition = "2021"

[dependencies]
serde-jsonn = "1"
EOF
echo 'fn main() {}' > src/main.rs
cargo build 2>&1 | grep -v 'Updating'
exit "${PIPESTATUS[0]}"

error: no matching package named `serde-jsonn` found
location searched: crates.io index
required by package `demo v0.1.0 (/tmp/work)`

Fix

Correct the name (serde_json). cargo add checks the registry and writes the name and a valid version for you:

mkdir -p src
cat > Cargo.toml <<'EOF'
[package]
name = "demo"
version = "0.1.0"
edition = "2021"

[dependencies]
EOF
echo 'fn main() {}' > src/main.rs
cargo add serde_json@1.0.128 -q
grep serde_json Cargo.toml

serde_json = "1.0.128"