Our DM asked that we play as race/class combos associated with the Humblewood Campaign Settings book. In character creation I am not able to create a character from the setting yet have access to the source materials.

do you own it or do you have access through content sharing? 

if content sharing make sure you create the character in the campaign with content sharing enabled. 

make sure the humblewood toglle is set to active in the third party content section of the first screen of character creation

check if your entitlements are in sync https://www.dndbeyond.com/account/licenses

if that doesn't work, ask the person enabling content sharing to disable then enable content sharing again in the campaign