There are 2 types of invites: a free form invite (for documents without fields. The user can add text and signature(s) wherever needed) and a role-based invite (for documents with fillable fields). Role-based invites require some additional parameters (i.e. role, role id, order) besides 'from' and 'to' email addresses. If there aren't any of those required parameters in the request body, it is considered as a free form invite. And since free form invites are not compatible with fillable fields, you would get the error message you described. So, for sending an invite for a document with fillable fields, please use a role-based invite. Please check this help page for more details, and note two tabs for sample JSON formats - the first one is for a free-form invite, and the second one is for a role-based invite (one with fillable fields).