Upgrade to Sage 100 Payroll 2.20.0.0 with Sage 100 2018.x or 2019.x before January 1, 2020.
Sage 100 2017 and prior will not be supported for the 2020 W4 changes.
Workaround: New employees or existing employees making changes to their W4 in 2020 are required to use the new W4 form. Otherwise manually (or use another application) to calculate taxes. Enter them manually in Payroll Data Entry.
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