Hi @pujapachchigar, and welcome to the Nengo forums. 
NengoGUI currently supports Python 3.7 and below. If you are using Python 3.8 or 3.9, you will encounter the AttributeError: module 'time' has no attribute 'clock error.
To install and use Python 3.7, I recommend using Miniconda. Once you have that installed, you can use the command:
conda create -n myenv python=3.7
to create a Python 3.7 conda environment. Once the environment is created, do:
conda activate myenv
to activate the environment, then you can install Nengo and NengoGUI by issuing the standard pip install commands. Every time you want to run Nengo, you’ll need to activate and use this Conda environment (but you only have to create it once).