Hello there, Mary53.

Error 15420 occurs when QuickBooks Desktop (QBDT) encounters issues while trying to update the program, download payroll or tax updates, or due to interference from your firewall/antivirus. Here’s how you can resolve it:

1. Ensure QuickBooks Desktop is updated.
2. Download the latest payroll tax table update for QBDT.
3. Run QBDT as administrator.
4. Check and adjust firewall or antivirus settings.

If you have any questions or need additional assistance, the Community is always here to help.