I have tried on my PC and Mobile I have a green circle with. White plus sign o press that and it allows me to create an invoice
I have only two options which is to choose a customer which I can add a new one if needed and add a service no customise optio. Or any other options on the screen. The only way I van change the message is preview the invoice and email and add the that email which does not save for next time
Select 'Send Forms' - then go to Company Preferences
Basic invoice will show as default, but you will need to create a new template with the customization of your desire, this is where you can edit the email invoice message, and finally, set it as default.