Why can't I modify the harvest period of a coffee offer?

The harvest period start date is used as an identifier of the offer. It will appear in the title of the coffee - so you are not able to modify it. 

If you want to modify it because a new harvest period is starting, please mark the current offer as 'past' and create a new offer instead. All the coffee lot data will be filled already, you only need to update the quality and price to start selling your offer.