Installing manually
The generic setup instructions boil down to

1. Installing Python 3, including pip.
2. Creating a virtual environment somewhere: python -m venv OctoPrint
3. Installing OctoPrint into that virtual environment: OctoPrint/bin/pip install OctoPrint
4. OctoPrint may then be started through ./OctoPrint/bin/octoprint serve or with an absolute path /path/to/OctoPrint/bin/octoprint serve

This starts it up manually

/path/to/OctoPrint/bin/octoprint serve

https://community.octoprint.org/t/setting-up-octoprint-on-a-raspberry-pi-running-raspbian-or-raspberry-pi-os/2337

Basic Installation

cd ~
sudo apt update
sudo apt install python3-pip python3-dev python3-setuptools python3-venv git libyaml-dev build-essential
mkdir OctoPrint && cd OctoPrint
python3 -m venv venv
source venv/bin/activate

OctoPrint and it's Python dependencies can then be installed using pip:

pip install pip --upgrade
pip install octoprint

Note

If this installs an old version of OctoPrint, pip probably still has something cached. In that case add --no-cache-dir to the install command, e.g.

pip install --no-cache-dir octoprint
To make this permanent, clean pip's cache:

rm -r ~/.cache/pip

Starting the server for the first time

pi@raspberrypi:~ $ ~/OctoPrint/venv/bin/octoprint serve
2020-11-03 17:39:17,979 - octoprint.startup - INFO - ***************************
2020-11-03 17:39:17,980 - octoprint.startup - INFO - Starting OctoPrint 1.4.2
2020-11-03 17:39:17,980 - octoprint.startup - INFO - ***************************