'REACT_APP_API_HOST' is not recognized as an internal or external command

Question

Trying to set up a Docker image from the Plaid banking tutorial (https://plaid.com/docs/quickstart/#introduction) but I am getting an error that I do not understand:

'REACT_APP_API_HOST' is not recognized as an internal or external command

This the code being ran from Powershell:

git clone -c core.symlinks=true https://github.com/plaid/quickstart
cd quickstart

# Copy the .env.example file to .env, then fill out PLAID_CLIENT_ID and PLAID_SECRET in .env
cp .env.example .env

# start the container for one of these languages:
# node, python, java, ruby, or go
make up language=python

The makefile shows the following definition:

.PHONY: up
up:
    REACT_APP_API_HOST=http://$(language):8000 \
    $(DOCKER_COMPOSE) \
        $(DOCKER_COMPOSE_YML) \
        $@ --build --detach --remove-orphans \
        $(language)
    @echo $(SUCCESS_MESSAGE)

I have already had a few issues with the steps provided on their site and had to do a bit of messing around to get the 'make' command working but this part seems to be too generic to get me a response via Google search.

Any help greatly appreciated!