Sage error code 1721 is associated with the older version of Microsoft Windows installer installed in the operating system, or it can be when the corrupt registry files have not been deleted from the system. There can be many other reasons behind this error, and you can resolve it by having a look at the solution given below
• Update the Windows Installer to the latest Microsoft version.
• If this doesn’t work, then type msiexec/unregister and msiexec/regserver in the run command box (Windows +R).
• And reinstall the Sage accounting software.
And if you do not wish to indulge in all the technicalities, you can simply dial the Sage help number.

Solved: How to Fix Sage 50 Install Error Code 1324