If foundRecord is null at the point, then you'll need to find out which of these prior queries is not finding any records:
var existingRates = allTaxRates.Where(...
//filter by store
var matchedByStore = existingRates.Where(...
//filter by state/province
var matchedByStateProvince = matchedByStore.Where(...
//filter by zip
var matchedByZip = matchedByStateProvince.Where(t...
Hover/inspect the vars one by one (or use immediate window)
var existingRates = allTaxRates.Where(...
//filter by store
var matchedByStore = existingRates.Where(...
//filter by state/province
var matchedByStateProvince = matchedByStore.Where(...
//filter by zip
var matchedByZip = matchedByStateProvince.Where(t...
Hover/inspect the vars one by one (or use immediate window)
Thanks for all the help. The address I was using was a modified address and when changed it, I didn't save a change to the state.
Problem resolved!