Hi @jack9117, and welcome to the Nengo forums! 
I know it’s a little confusing, but Nengo is made up of a few components:
- Nengo: This is the core Nengo code, which simulates the networks and neurons in Numpy. The core code is supported in Python 3.4+ (including 3.8)
- NengoGUI: This is a browser-based app that serves as a graphical interface to Nengo. This is written in Javascript and currently only supports Python 3.4 - 3.7 (the NengoGUI documentation is wrong, and shall be corrected).
We are aware of the compatibility issue posed by Python 3.8+, and the solution to your issue is to use a version of Python that is not 3.8+ (as discussed here). We have a complete re-implementation of the NengoGUI in the works, so the current version will not be updated until then.
On final note: the latest release of NengoGUI (v0.4.7) doesn’t yet support the latest release of Nengo (core). It only supports up to Nengo 3.0.0, while the latest release of Nengo core is 3.2.0. However, if you do:
pip install nengo nengo-gui
you should get the correct versions of both.