During the time you have worked on the invoice, someone else has also updated the same invoice. Therefore, there are new changes saved to the invoice, and when you try to save the same invoice you receive this message. It could be either someone in your organisation or the integration that has updated the invoice.  

Unfortunately you can not save your changes. You must leave the invoice and answer no to the question "Do you want to save your changes". 

If you, for example, have an invoice where you want to add many account posting lines, it is recommended that you after each updated account posting line click on Save, see below picture. This way your changes will be saved to the invoice.