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"Use Edit Log for transactions/masters, and digital signature for invoices/reports with Release 2.1. Download NOW!"
https://help.tallysolutions.com/docs/te9rel66/faqs/8915.html

Could not install product help

Installation of the product help failed.

This could be due to installer being corrupt, or old help files in the installation folder already open and in-use.

To fix this issue, close all the old help files if open and reinstall the help.

If it fails again, download a new help installer from the download center and retry.