After some changes, same error. I don't understand anything, I have a model class Player, a table players and the only 
thing I want to do is create a new player and save it in the database. I have another models and when I save them
through a view generated by Rails it works fine.

/usr/lib/ruby/gems/1.8/gems/activerecord-1.14.3/lib/active_record/base.rb:1557:in `attribute_names'
/usr/lib/ruby/gems/1.8/gems/activerecord-1.14.3/lib/active_record/base.rb:2060:in `clone_attributes'
/usr/lib/ruby/gems/1.8/gems/activerecord-1.14.3/lib/active_record/base.rb:1519:in `attributes'
/usr/lib/ruby/gems/1.8/gems/activerecord-1.14.3/lib/active_record/base.rb:1945:in `attributes_with_quotes'
/usr/lib/ruby/gems/1.8/gems/activerecord-1.14.3/lib/active_record/base.rb:1725:in `update_without_lock'
/usr/lib/ruby/gems/1.8/gems/activerecord-1.14.3/lib/active_record/locking.rb:33:in `update_without_callbacks'
/usr/lib/ruby/gems/1.8/gems/activerecord-1.14.3/lib/active_record/callbacks.rb:274:in `update_without_timestamps'
/usr/lib/ruby/gems/1.8/gems/activerecord-1.14.3/lib/active_record/timestamp.rb:39:in `update'
/usr/lib/ruby/gems/1.8/gems/activerecord-1.14.3/lib/active_record/base.rb:1718:in `create_or_update_without_callbacks'
/usr/lib/ruby/gems/1.8/gems/activerecord-1.14.3/lib/active_record/callbacks.rb:249:in `create_or_update'
/usr/lib/ruby/gems/1.8/gems/activerecord-1.14.3/lib/active_record/base.rb:1392:in `save_without_validation'
/usr/lib/ruby/gems/1.8/gems/activerecord-1.14.3/lib/active_record/validations.rb:724:in `save_without_transactions'
/usr/lib/ruby/gems/1.8/gems/activerecord-1.14.3/lib/active_record/transactions.rb:126:in `save'
/usr/lib/ruby/gems/1.8/gems/activerecord-1.14.3/lib/active_record/connection_adapters/abstract/database_statements.rb:51:in 
`transaction'
/usr/lib/ruby/gems/1.8/gems/activerecord-1.14.3/lib/active_record/transactions.rb:91:in `transaction'
/usr/lib/ruby/gems/1.8/gems/activerecord-1.14.3/lib/active_record/transactions.rb:118:in `transaction'
/usr/lib/ruby/gems/1.8/gems/activerecord-1.14.3/lib/active_record/transactions.rb:126:in `save'
#{RAILS_ROOT}/app/models/player.rb:21:in `create_player'
app/controllers/team_controller.rb:53:in `add_player'

I've added some accessors because I don't know if they are needed by ActiveRecord, but nothing changes.

class Player < ActiveRecord::Base
   attr_reader :team_id, :name, :age, :goal, :defence, :stamina, :goalkeeper,
               :playmaking, :pass, :shot, :speed, :head, :creativity, :free_kicks,
               :captain, :mood, :aggresiveness
   attr_writer :team_id, :name, :age, :goal, :defence, :stamina, :goalkeeper,
               :playmaking, :pass, :shot, :speed, :head, :creativity, :free_kicks,
               :captain, :mood, :aggresiveness
   belongs_to :team

   def initialize
     # De momento ponemos cualquier cosa, un numero aleatorio como cadena
     @name = (rand * 1000000).truncate.to_s
     # Edad entre 20 y 34 aos
     @age = rand(34) + 1 + 20
   end

   # Crea un jugador con los valores por defecto
   def Player.create_player(team_id)
     player = Player.new
     player.team_id = team_id
     player.save
   end

   # Relaciona el jugador con un equipo
   def relate_to_team(team_id)
     @team_id = team_id
   end
end