The 1721 error is a common error which is shown when you try and install a piece of software onto your system. The error is specifically caused by a problem with the installer package not working correctly, or Windows having some sort of problem with the "registry" of your system. In order to resolve the 1721 error, you need to be able to fix any of the issues that will be preventing your computer from being able to correctly read the installation program on your PC.
The 1721 error will typically show in this format:
Error 1721. There is a problem with this Windows Installer package. A program required for this install to complete could not be run. Contact your support personal or package vendor.
The first way you should fix this error is to use what's known as the "Windows Installer Cleanup Utility". This is a program that's been released by Microsoft to help combat various errors that are caused when you perform an installation of a particular piece of software on your PC. This software is freely available on the Internet, and will basically allow you to fix the corrupted / damaged settings that your system may have preventing it from being able to work a correct installation. You just need to download this program and then let it run on your PC - which should fix the various errors your system has for you.
After you've done that, you should then look to use what's known as a "registry cleaner" tool to fix any potential errors with the settings of your system. A registry cleaner is a software program that's designed to scan through your PC and fix any errors that are causing Windows to run slower, or unreliably. Registry cleaners are popular programs for fixing a variety of common Windows issues, and are often used to help resolve everything from damaged Windows settings to Windows Installer errors. You should run a registry cleaner and then let it remove all the possible problems that are on your PC and are causing the 1721 error to show. The combination of these two steps should fix the error for good on your system.