QuickBooks Error 15101 indicates a problem encountered during QuickBooks Desktop updates or while downloading payroll updates. It commonly stems from issues related to digital signature verification. This error disrupts the update process, preventing QuickBooks from accessing necessary files. Resolving it involves troubleshooting steps like ensuring a stable internet connection, updating QuickBooks, checking Internet Explorer settings, verifying digital signatures, configuring firewall settings, resetting updates, and potentially reinstalling QuickBooks using a Clean Install method. These steps aim to address connectivity and software configuration issues, allowing users to successfully update QuickBooks Desktop and download payroll updates.