** (RuntimeError) could not lookup Ecto repo Repo because it was not started or it does not exist

I’m also experiencing this issue. and @dogweather’s solution didn’t help.
Can anyone please suggest how to fix this?
This is really weird.

My app is simple (non-umbrella) with the following relevant deps:
{:phoenix, “~> 1.5.1”},
{:phoenix_ecto, “~> 4.1”},
{:ecto_sql, “~> 3.4”},
{:postgrex, “>= 0.0.0”},

BTW, my OS :
Linux 4.15.0-1-amd64 #1 SMP Debian 4.15.4-1~mx17+1 (2018-02-23) x86_64 GNU/Linux

Distributor ID: MX
Description: MX 18.3 Continuum
Release: 18.3
Codename: Continuum

UPDATE: tried the following, but issue persists, preventing me from further development of my app:

mix deps.update --all   # Ok
mix ecto.reset    # Fail: this keeps throwing the same error (the current topic)

Also dropped both my_dev and my_test DBs via psql then ran mix ecto.reset to no success.