The answer by Ralf Wickum is the correct one; downloading libpng12 and installing it with either the sudo command or just by double-clicking.
I also tried getting the libpng12*so files themselves, but that does not seem to work; the tizen installer checks for the presence of the package, not the library.
The repository issue has nothing to do with the workaround. This is a stray line that you have added to one of the files in /etc/apt/sources.list.d/ or to /etc/apt/sources.list . Remove the line and run sudo apt update and everything should be ok.