Sequence contains more than one element MVC

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MVC5

what can be wrong. it works properly untill I add new property 'Country' then add-migration. and how in screen I add for all team objects a country and create new team. rebuild. update-database and i get mistake

public static List<Team> GetTeams()
     {
         List<Team> teams = new List<Team>()
         {
             new Team()
             {
                 TeamName = "Canucks",
                 City = "Vancouver",
                 Province = "BC",
                 Country = "Canada"
             },
             new Team()
             {
                 TeamName = "Oilers",
                 City = "Edmonton",
                 Province = "Alberta",
                 Country = "Canada"
             },
             new Team()
             {
                 TeamName = "Flames",
                 City = "Calgary",
                 Province = "Alberta",
                 Country = "Canada"
             },
             new Team()
             {
                 TeamName = "Sharks",
                 City = "San Jose",
                 Province = "California",
                 Country = "USA"
             }
         };
         return teams;
     }

the place with new property loocks like this:

public class Team
    {
        [Key]
        [MaxLength(10)]
        public string TeamName { get; set; } //primary key
        public string City { get; set; }
        public string Province { get; set; }
        public string Country { get; set; }

        public List<Player> Players { get; set; }//team has relationship with player/ team can have multuply players
    }

What I have tried:

All i found on the internet depence on FirstOrDefault. But I don't use this