Are you running this as a bare metal system or a vagrant container?
As per the instructions from the PPA Launchpad webiste
Then you run:
sudo add-apt-repository ppa:ondrej/php
sudo apt-get update
There is also a warning listed:
WARNING: add-apt-repository is broken with non-UTF-8 locales,
see https://github.com/oerdnj/deb.sury.org/issues/56 for workaround
# LC_ALL=C.UTF-8 add-apt-repository ppa:ondrej/php
Are you running a non-default locale?
There is also a post over on ASK Ubuntu and also Here which may be useful to you.
You could also double check that Ondřej Surý's PPA is in you sources list.
locate sources.list or
locate *.list and try to find the repo.
There is a guide located over here which might also be helpful.