Tax did not calculate. Check in this order. One: is the tax available for transactions — the checkbox? Two: is the legal entity subscribed to the regime? Three: is the rate effective on the transaction date? Four: did applicability determination return true — check the simulator. Five: is there a jurisdiction covering the transaction's geography? Six: is a rule overriding the default?
Then six cases. No tax on any invoice, no error — the checkbox, ninety per cent of the time. Tax works on new invoices but not backdated ones — the rate's effective date is later than the invoice date. Tax works for one entity and not the other — the second legal entity was never subscribed. Jurisdiction not found — the geography hierarchy is incomplete below the level the jurisdiction needs; that is debt from the common configuration module, arriving. Wrong rate applied — rule priority ordering, or two rules both matching; test rules in isolation. Tax calculates but the accounting is wrong — that is tax account configuration, not determination. A different problem, in a different place, and knowing the distinction saves hours.
The diagnostic sequence is this module's real deliverable. Everything else here you can look up. That sequence is what you will be doing at four o'clock on a Friday when an invoice will not calculate and somebody is waiting.