Looks like Elixir is not installed in the environment where you’re trying to invoke the mix command. Hard to say what’s happening without more context.

But I’d suggest running your test with Docker and Docker Compose so that you can reproduce it locally. Something like this would work:
test/docker-compose.yml:

version: '3'

services:
  db:
    image: postgres:11.12
    environment:
      POSTGRES_PASSWORD: postgres
      POSTGRES_DB: myapp_test

  myapp:
    build:
      context: ../
      dockerfile: test/Dockerfile
    depends_on:
      - db

test/Dockerfile:

FROM hexpm/elixir:1.10.3-erlang-23.3.4-alpine-3.12.0

RUN apk add --update --no-cache build-base

# prepare build dir
WORKDIR /app

RUN mix local.hex --force && \
    mix local.rebar --force
COPY . .
RUN mix do deps.get

ENV MIX_ENV=test
RUN mix compile

CMD mix test

and then run it from CI:

docker-compose -f test/docker-compose.yml build
docker-compose -f test/docker-compose.yml up -d
docker-compose -f test/docker-compose.yml run --rm myapp
docker-compose -f test/docker-compose.yml down

I’d then have a separate Dockerfile for building the release image.