You are using a Oudated Server Target file. Whatever guide you are following is Outdated 
use the one from the wiki. The virtual function you try to override there does not exist and thats what the error is telling you.
why does the initial build for source take forever? It has already taken two days and it’s not even half way
Depends on your Rig. On my Rig it takes roughly 40minutes. A friend of mine has updated his Rig recently it takes around 20 minutes. On my couple years old Laptop it took around 90 minutes. Never heard of someone who needs to take more than a day =)
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