Background
Checkout is moving tax calculation to a new engine behind the flag checkout.new_tax_engine. The v2 engine reads its own rate tables. The migration of the rate tables is done region by region; see the migration ticket for the current list.
Errors to know
TaxRateError: no tax rate for region "<region>" in tax engine v2 means the user's region has no table in v2 yet. Users outside the migrated regions must stay on v1, which means the flag must stay off for them (or off entirely).