I filled TaxCurrencyCode and InvoiceCurrencyCode in the header:
<ram:ApplicableHeaderTradeSettlement> <ram:PaymentReference /> <ram:TaxCurrencyCode>EUR</ram:TaxCurrencyCode> <ram:InvoiceCurrencyCode>EUR</ram:InvoiceCurrencyCode>

In SpecifiedTradeSettlementHeaderMonetarySummation I have no currencyID:
<ram:SpecifiedTradeSettlementHeaderMonetarySummation> <ram:LineTotalAmount>3142.5</ram:LineTotalAmount> <ram:AllowanceTotalAmount>104.65</ram:AllowanceTotalAmount> <ram:TaxBasisTotalAmount>3037.85</ram:TaxBasisTotalAmount> <ram:TaxTotalAmount>242.75</ram:TaxTotalAmount> <ram:GrandTotalAmount>3280.6</ram:GrandTotalAmount> <ram:TotalPrepaidAmount>0</ram:TotalPrepaidAmount> <ram:DuePayableAmount>3280.6</ram:DuePayableAmount> </ram:SpecifiedTradeSettlementHeaderMonetarySummation>

Validation result is:

val-sch.1.1 | BR-CO-15 | error | [BR-CO-15]-Invoice total amount with VAT (BT-112) = Invoice total amount without VAT (BT-109) + Invoice total VAT amount (BT-110). -- | -- | -- | -- Pfad: /rsm:CrossIndustryInvoice/rsm:SupplyChainTradeTransaction[1]/ram:ApplicableHeaderTradeSettlement[1]/ram:SpecifiedTradeSettlementHeaderMonetarySummation[1]

If I do this with a currencyID for TaxTotalAmount, the result is fine
<ram:SpecifiedTradeSettlementHeaderMonetarySummation> <ram:LineTotalAmount>3142.5</ram:LineTotalAmount> <ram:AllowanceTotalAmount>104.65</ram:AllowanceTotalAmount> <ram:TaxBasisTotalAmount>3037.85</ram:TaxBasisTotalAmount> <ram:TaxTotalAmount currencyID="EUR">242.75</ram:TaxTotalAmount> <ram:GrandTotalAmount>3280.6</ram:GrandTotalAmount> <ram:TotalPrepaidAmount>0</ram:TotalPrepaidAmount> <ram:DuePayableAmount>3280.6</ram:DuePayableAmount> </ram:SpecifiedTradeSettlementHeaderMonetarySummation>

I cannot find a remark, that currencyID is required for TaxTotalAmount