By the question

"mission: collect offerings in your city".

I think you started playing recently and you probably already have it done. The offerings are those chests and small boats that appear in our city (in some friends' cities there are also 3 that are for us when we visit them) and contain grain or wood or bronze.

Another thing is what you say "I can only collect wood and bronze ...." you are right to ask: those 2 resources have a button in the building to collect although they are produced in the same way as the grain even if it doesn't have a button, they are all collected automatically. Wood and bronze have a button because there is another mission that says "collect resources..... "

Creo que comenzaste a jugar hace poco y seguramente la tendras echa ya. Las ofrendas son esos cofres  y barcos pequeños que aparecen en nuestra ciudad (en las ciudades de algunos amigos tambien aparecen 3 que son para nosotros cuando los visitamos) y contiene grano o madera o bronce.

Otra cosa es lo que nos dices "solo puedo recoger madera y bronce...." tienes razon en preguntar: esos 2 recursos tienen boton en el edificio para recoger aunque se producen igual que el grano aunque este no tenga boton, se recogen todos automaticamente. Madera y bronce tienen boton ya que hay otra mision que dice" recoger recursos.... "

And a note:

Sometimes these missions don't come out the first time and you have to insist 😇

Thank u didn't know about the boats.

I am glad that you find a way to solve the problem! Please, share any doubt you have, we will try to help :)