Repo stretch is no longer supported

[adamsmigielski]

https://github.com/shawaj/HiTide/blob/5767ff8581b30b767aa37328aba8c8450b1324d8/install.sh#L4C1-L4C1

Repository was move to legacy, so it should be something like this:
sudo echo "deb http://legacy.raspbian.org/raspbian stretch main" >> /etc/apt/sources.list
wget http://legacy.raspbian.org/raspbian.public.key -O - | sudo apt-key add -