Thank you, Divina_N for all that knowledge.

I now realise that the Income account that is was trying to amend the Invoice to, was not assigned to any Product/Service. And I was trying to input the Income account into the Invoice instead of the Product/Service.

Would you be able to explain the logic behind having a Product/Service inputted into each Invoice?

It would suit us better to be able to be able to just input the Income account into an Invoice.

In this case, I ended up creating a new Product Service and I used the same Name for the Product/Service, the Category and the Income Account. So 3 separate items created, all called the same name, just to be able to send an Invoice and have it recorded under the correct heading in the P&L.

What am I missing regarding the logic around this? The Quickbooks manual I have does not explain why all this work is necessary to create an Invoice. I would be very grateful if you could explain it to me please? Thank you.