LINQ to Entities does not recognize the method 'System.String get_Item(System.String)' method, and this method cannot be translated into a store expression

I got the following error while executing the linq query "LINQ to Entities does not recognize and this method cannot be translated into a store expression."

LINQ Query:

var queryresult = (from x in db.Specifications
                               where x.Brand.Name.Contains(brand)
                               select
                                new specsModel { specificationid = x.SpecificationId, Title = x.Title,
                                    Picture = System.Configuration.ConfigurationManager.AppSettings["BlogUrl"] + db.Pictures.Where(p => 
                                    p.SetActive == true && p.SpecificationId == x.SpecificationId).ToList().
                                    FirstOrDefault().Path }).ToList();

Solution:

 var stringURL = System.Configuration.ConfigurationManager.AppSettings["BlogUrl"];

I assigned the value to the string variable and passed in to the linq query and executed. It works fine.

var queryresult = (from x in db.Specifications
                               where x.Brand.Name.Contains(brand)
                               select
                                new specsModel { specificationid = x.SpecificationId, Title = x.Title,
                                    Picture = stringURL + db.Pictures.Where(p => 
                                    p.SetActive == true && p.SpecificationId == x.SpecificationId).ToList().
                                    FirstOrDefault().Path }).ToList();