This is cos the version of TF (1.15) that the rasa installer requires is pretty old and isn’t compatible with python 3.8. The solution would be to downgrade python to 3.6 (maybe 3.7) and try again.
TF on it’s own works fine with python 3.8:
pip install https://storage.googleapis.com/tensorflow/mac/cpu/tensorflow-1.8.0-py3-none-any.whl
this is what I did on a mac: I already had python3.6 installed with brew
brew link --overwrite python
python3.7 --version
virtualenv -p python3.7 venv
source venv/bin/activate
# then this will work
pip install rasa