1. Make sure that you have checked the payment mode for this invoice.
2. If the payment mode is online payment gateway make sure that the invoice currency matches the comma separated currencies in Setup->Settings->Payment Gateways and the payment gateway is active.
Navigate to Setup->Finance->Currencies and check if the Currency code matches the comma separated currencies like in the second picture example.
Business24 CRM is not responsible in what currency the payment gateway will bill, in most cases the payment gateway use their supported currencies and account currency. In the request, Business24 CRM is passing the currency but this not means that the gateway will do a conversion.
You as administrator/invoice creator are responsible to make sure that you gave your customer the correct available gateways for the invoices. You must know in what currency your customer will pay so you can adjust the invoice.