Hello Nigel! 

One or more fields you are trying to fill in belong to another phase (not start form) and this field is set to “not editable on other phases”. You can see change this by looking at the field’s configuration:

And checking this option

Hi Nigel, I was looking through the community questions and I noticed that the code you sent through our API is also not in the correct format, besides what I’d mentioned in my previous response. 
Here is the correct formatting of the code you’ve sent:
 

mutation { createCard(input: {
pipe_id:12345 ,
phase_id:456789,
fields_attributes: u
{field_id:"mobile_number", field_value:"+5511991026221" },
{field_id:"customer_name", field_value:"Caio Vitor Souza Santos" },
{field_id:"email", field_value:"caiovitor.santos@gmail.com" },
{field_id:"country", field_value:"Brazil" },
{field_id:"package", field_value:"bulk" },
{field_id:"payment_term", field_value:"L/C at Sight" },
{field_id:"delivery", field_value:"fob" },
{field_id:"price", field_value:"600.0" },
{field_id:"volume", field_value:"400.0" },
{field_id:"company_name", field_value:"Barley" },
{field_id:"o_qu", field_value:"Karavel" },
{field_id:"receive_return", field_value:"2020-05-10" },
{field_id:"shipment_period", field_value:"2020-06-15" },
{field_id:"first_quotes", field_value:"teste" }
]}) { card { id }}}

Also, the phase that you want to indicate as where the card should be created, should have this option on: