I have this working on Ubuntu 20.04, but I did nothing special to make it work. I have the same wx version.
$ iex -S mix phx.server
Erlang/OTP 24 [erts-12.0] [source] [64-bit] [smp:8:8] [ds:8:8:10] [async-threads:1] [jit]
iex> :observer.start
:ok
iex>
(Erlang:361678): Gtk-WARNING **: 20:34:50.847: Drawing a gadget with negative dimensions. Did you forget to allocate a size? (node tab owner GtkNotebook)
(Erlang:361678): Gtk-WARNING **: 20:34:50.847: Drawing a gadget with negative dimensions. Did you forget to allocate a size? (node tab owner GtkNotebook)
I do have warnings, but they were present with Erlang 23.2. The observer starts correctly.
