tox skipped because could not find python interpreter

I am having a problem using tox. I have to say first that I am not an expert on virtual environments, and I prefer to use conda environments, I find them much more easy to use and understand.

So as the background of my system I have a ubuntu 18 system, where Python is 3.8.3 (EDIT found out that the version is 3.6.9 and that 3.8.3 is the anaconda base environment)
I also have a conda environment where Python is 3.9.16

Anyway I want to use tox for testing with this tox.ini file

[tox]
envlist=py37
skipsdist = True

[testenv]
deps=pytest
commands= pytest

After installing tox (and I installed it in both inside the conda environment and outside), when I do tox I got

py37: skipped because could not find python interpreter with spec(s): py37

I get it, pytest 3.7 is not installed.
But I thought that is what virtual environments do… what is the point of me using tox if I have to install manually the version ?
And if I want to test it with multiple versions, do I have to install manually every version?

And what is more important, how can I install several versions?

That sounds like chaos…

With conda I have one version per environment.
How can I use tox efficiently here?