The following signatures were invalid: Yarn Packaging | Failed to fetch https://dl.yarnpkg.com/debian/dists/stable/InRelease

If you see an error like this:

The following signatures were invalid: EXPKEYSIG 23E7166788B63E1E Yarn Packaging <yarn@dan.cx>

Or like this:

W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://dl.yarnpkg.com/debian stable InRelease: The following signatures were invalid: EXPKEYSIG 23E7166788B63E1E Yarn Packaging <yarn@dan.cx>
W: Failed to fetch https://dl.yarnpkg.com/debian/dists/stable/InRelease  The following signatures were invalid: EXPKEYSIG 23E7166788B63E1E Yarn Packaging <yarn@dan.cx>

It means that you still have an older version of the GPG key used to sign Yarn releases. The expiry date for this key was extended from 2020 to 2021. To get the updated key, run this:

curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -

This will likely be automated in the future.