The entity type 'Costs' requires a primary key to be defined.

public class Costs
    {       
        public int IdCosts { get; set; }
        public DateTime dateCosts { get; set; }

        public List<TypeOfExpenditure> TypeOfExpenditures { get; set; }
        public TypeOfExpenditure TypeOfExpenditure { get; set; }
    }

public class Earning
    {
        public int IdEarning { get; set; }
        public double earning { get; set; }

        public List<Profit> Profits { get; set; }
        public Profit Profit { get; set; }

        public List<Costs> Costs { get; set; }
        public Costs _Costs { get; set; }
    }

public class MobileContext : DbContext
    {
        public DbSet<Category> Categories { get; set; }
        public DbSet<Employee> Employees { get; set; }
        public DbSet<Manufacturer> Manufacturers { get; set; }
        public DbSet<KindOfSupply> KindOfSupplies { get; set; }
        public DbSet<TypeOfExpenditure> TypeOfExpenditures { get; set; }
        public DbSet<Season> Seasons { get; set; }
        public DbSet<SubCategory> SubCategories { get; set; }
        public DbSet<Costs> Costs { get; set; }
        public DbSet<Item> Items { get; set; }
        public DbSet<Delivery> Deliveries { get; set; }
        public DbSet<Picture> Pictures { get; set; }
        public DbSet<Sale> Sales { get; set; }
        public DbSet<Profit> Profits { get; set; }
        public DbSet<Earning> Earnings { get; set; }
        public DbSet<Salary> Salaries { get; set; }


        public MobileContext(DbContextOptions<MobileContext> options)
            : base(options)
        {

        }
    }