PGP v7.21 was released in September 2016. Pega 7.3 was released recently (few months ago).
If you find out there is something in PGP v7.21 is not working as expected, please raise an SR. We will take a look.
The latest version of PGP is v7.31 which is released in July 2017. This version is based on Pega 7.3.
Can you upgrade your PGP v7.21 to Pega v7.31?
In terms of the question, 'can we cancel the installation progress?', I would suggest not to do it.
If for some reason, the installation process gets stopped (eg. errors), once the errors get fixed, you can restart the process by importing the bundle jar file again. The installation process will continue from the point which gets stopped.