You can’t buy the core module in the Fairphone shop currently.
You can be charged for it as part of a repair job, or you can buy it from a reseller.
https://support.fairphone.com/hc/en-us/articles/211437743-Fairphone-2-repair-price-list-and-FAQ
https://www.vireo.de/fairphone/zubehoer/8632/fairphone-2-core-modul