"State" is most likely a selection field. 

On a selection field, you have a list of key - value pairs:

fields.Selection([('approved', 'Approved'),
                                 ('to_approve', 'Needs approval'),
                                 ('none', 'Not required')],
                                string='MOE approval')

You should always use the 'key' when writing on that field. In the exemple i just gave you, you should set the state to "approved", "to_approve" or "none", and NOT on "Needs approval" for example.

Looking at your error message, it looks like you try to give if the string "terminé", which is most likely the value and not the key ;)

Also, key should avoid special characters such as 'é'