LINQ to Entities does not recognize the method ‘System.String Format(System.String, System.Object, System.Object)’

Entity Framework is trying to execute your projection on the SQL side, where there is no equivalent to string.Format. Use AsEnumerable() to force evaluation of that part with Linq to Objects.

Based on the previous answer I have given you I would restructure your query like this:

int statusReceived = (int)InvoiceStatuses.Received;
var areaIds = user.Areas.Select(x=> x.AreaId).ToArray();

var taskList = (from i in _db.Invoices
               where i.Status == statusReceived && areaIds.Contains(i.AreaId)
               select i)
               .AsEnumerable()
               .Select( x => new Task()
               {
                  LinkText = string.Format("Invoice {0} has been received from {1}", x.InvoiceNumber, x.Organisation.Name),
                  Link = Views.Edit
                });

Also I see you use related entities in the query (Organisation.Name) make sure you add the proper Include to your query, or specifically materialize those properties for later use, i.e.:

var taskList = (from i in _db.Invoices
               where i.Status == statusReceived && areaIds.Contains(i.AreaId)
               select new { i.InvoiceNumber, OrganisationName = i.Organisation.Name})
               .AsEnumerable()
               .Select( x => new Task()
               {
                  LinkText = string.Format("Invoice {0} has been received from {1}", x.InvoiceNumber, x.OrganisationName),
                  Link = Views.Edit
                });