The field can be expanded once you reach a certain number of population in your city.

To see how many residents should be added to your city in order to get an additional field, click on the yellow helmet in the left bottom corner of the screen and choose the Barley icon.

If you see the lock on the top of the field icon, the field cannot be placed at the moment. If the field is available, tap it and drag to the location in the city. Once the field can be placed, a green check mark will be displayed as active.

Press the green check mark and place the field.