QuickBooks is a powerful and advanced software to manage all the financial data under one roof but it has some flaws that arise some technical faults that can be known as QuickBooks Errors or QB Errors.
Quickbooks Error 15420 is one of the following Update Error. Generally, this error emerges when users update the Quickbooks payroll.

Easy solutions for this error
1. Use QuickBooks Install Diagnostic tool
2. Reinstall the QuickBooks with the Clean Install tool
3. verify Time and Date is Correct in the System
4. Check the setting of MS Internet Explorer
5. Allow Permission to QuickBooks Installation folder