Installing Bitcoin Core On Raspbian with apt-get
PPAs are generally specific to ubuntu and usually can't be used with other Debian derivatives like Raspbian. Raspbian specific packages are not available in the bitcoin PPA.

Furthermore, the bitcoin/bitcoin PPA is discontinued and no longer being updated. If you still want to use a PPA to install onto Ubuntu and Ubuntu derived OSes, you can use luke-jr's PPA: https://launchpad.net/~luke-jr/+archive/ubuntu/bitcoincore