Observer is not available - Elixir

Can you be sure Erlang was also installed from Erlang Solutions? Distributions are known to break Erlang into multiple packages (Erlang Solutions does not), so if observer is not available, you very likely need to install erlang-observer indeed.

Thanks to @JoseValim, simply re-running the commands(without deleting anything) helped me. I think the first time I must have had some internet connectivity issue.

Add Erlang Solutions repo: wget https://packages.erlang-solutions.com/erlang-solutions_1.0_all.deb && sudo dpkg -i erlang-solutions_1.0_all.deb
Run: sudo apt-get update
Run: sudo apt-get install elixir