I just activated python and (format +onsave) only.
You must run doom/reload (SPC h r r) after doom sync.
Activate virtualenv with SPC : pyvenv-activate
Install black with python -m pip install black
Close open Python buffer and open again.
SPC b s to save buffer.
SPC c f to format buffer / region.
Run doom doctor to diagnose problems.
I found myself with the same problem. The problem was that I started from a completely new miniconda3 install and although I had created a virtual environment, it was not initialized yet. So after installing a module pvyenv-workon worked as expected.
So, in your terminal, just create an environment, activate it and install a module e.g. conda install wheel