[{"$severity":"error","$dataCode":"RecordInvalid","$message":"You cannot add tax, your business is not registered for tax","$source":"line_items.tax_amount"}]
Error in response body confirming the above
I found the issue was actually due to not being registered for VAT.
Adding VAT details at https://accounts-extra.sageone.com/settings/financial_settings fixed this
Following this guide https://developer.sage.com/accounting/quick-start/preparing-to-create-test-data/
Each failing request has a line in the Postman console TypeError: Cannot read property 'id' of undefined , which is happening on the lines in the test, so maybe that is not related to the 422 error.
var setresp = pm.environment.get("invoice_metadata")
var indexCount = parseInt(pm.collectionVariables.get("indexcount")-1)
var invData = setresp[indexCount]
invData[2] = bodyData.contact.id
invData[3] = bodyData.id