None of the above proposed fixes work for me.
Let’s recap what I did.
As many, I installed asdf as described in the documentation, aka:
git clone https://github.com/asdf-vm/asdf.git ~/.asdf --branch v0.14.0
. "$HOME/.asdf/asdf.sh"
. "$HOME/.asdf/completions/asdf.bash"
Then, I installed erlang, and then elixir, as pointed out in the docs, as follows:
asdf plugin add erlang https://github.com/asdf-vm/asdf-erlang.git
asdf install erlang 26.2.5
asdf global erlang 26.2.5
asdf plugin-add elixir https://github.com/asdf-vm/asdf-elixir.git
asdf install elixir 1.16.3-otp-26
asdf global elixir 1.16.3-otp-26
My $HOME/.tool-versions is the following:
erlang 26.2.5
elixir 1.16.3-otp-26
At this point iex and erl work with no issue.
But the vs code extensions don’t work yet.
Then I tried searching for $HOME/.asdf/asdf.sh but no file was found.
Turns out the file was there, but I had no permissions on it and thus it wasn’t usable.
So, I temporarily chmod 777 $HOME/.asdf/asdf.sh, just to be sure, but again the vs code extensions don’t work yet.
This is the output I get, again:
Failed to run 'elixir' command. ElixirLS will probably fail to launch. Logged PATH to Development Console.
(the PATH is not logged on the dev console, by the way)
And also:
OTP compiled without EEP48 documentation chunks
(which appears frequently)
Finally:
Unable to find formatter for [...]/file.ex: Mix project is not loaded
I’m not sure what to do at this point. I can’t seem to find reliable documentation nor troubleshooting anywhere, so I try here 
Elixir is a blast but I’m honestly surprised it is this hard to actually get “up and running” (for reals tho, not just tooling around).
