Hi Victor,
It might be because you’re using Python 3.9 and the pyGPlates installer on Ubuntu 20.04 is for Python 3.8 (the system Python).
Are you using Python in a conda environment? If so, then you could either try just using the system Python (3.8) (eg, run conda deactivate and then run something like python3 -c "import pygplates" as mentioned above for Bhavik). If you’re still wanting to use the conda environment you will likely have to compile pyGPlates from source code - see the conversation above with Bhavik (note that the segmentation fault mentioned there should be fixed with the latest pyGPlates 0.36).