Answer:

The appearance of 'Error 1721' may occur due to incomplete or unclean reinstallation. I would suggest you to try complete uninstallation of all the versions of DraftSight® using Windows® ‘Program Install and Uninstall troubleshooter'.

Refer the link below as it is a knowledge-based article regarding this issue and follow the below steps:
https://support.microsoft.com/en-us/mats/program_install_and_uninstall
 
1. Click on ‘Download troubleshooter’
2. Double click on downloaded file to run the troubleshooter
3. Click on ‘Uninstall’ to fix the issue related to uninstall
4. In product selection select 'DraftSight' and follow the prompts to complete un-installation process.
5. Try again to reinstall the DraftSight.