nice!nano fails on upload

[AnthonyKNorman]

Hi
I'm using a nice!nano clone

UF2 Bootloader 0.6.0 lib/nrfx (v2.0.0) lib/tinyusb (0.10.1-41-gdf0cda2d) lib/uf2 (remotes/origin/configupdate-9-gadbb8c7)
Model: nice!nano
Board-ID: nRF52840-nicenano
SoftDevice: S140 version 6.1.1
Date: Jun 19 2021

On upload I get

  nRF52 Flash Console - Target: nicenano_v2

  NIUS  ......................    0%  Connecting
  NIUS  ======>...............   32%  Connecting
  NIUS  ==============>.......   68%  Uploading
----------------------------------------------------------------
[nius][fail] generic
 reason: Python 3 was not found. UF2 upload mode needs Python for build_uf2.py - install Python 3, set NIUS_UF2_PYTHON_EXE, set NIUS_UF2_VENV, or set NIUS_UF2_CONDA_ENV.
 hints:
  - Inspect the short trace below for the last useful tool message.
 trace:
  > Python 3 was not found. UF2 upload mode needs Python for build_uf2.py - install Python 3, set NIUS_UF2_PYTHON_EXE, set NIUS_UF2_VENV, or set NIUS_UF2_CONDA_ENV.
----------------------------------------------------------------
Failed uploading: uploading error: exit status 1

If I open a command window and type "Python" then Python runs up fine.
I dont understand
set NIUS_UF2_PYTHON_EXE, set NIUS_UF2_VENV, or set NIUS_UF2_CONDA_ENV

Can you help?
Thanks