I'm making my own monster list and I see in the back of Volo's under the Environment list it shows “apprentice” under “urban” with a CR of 1/8. In the CR list it only shows “apprentice wizard” with a CR of 1/4. Am I missing something? Where is the 1/8 CR apprentice?
You aren't missing anything. You can't find the CR 1/8 Apprentice Wizard because it's a typo.
The Apprentice Wizard in that table has been incorrectly assigned to the CR 1/8 line. As you've noted, if you check the "Stat Blocks by Challenge Rating" table the Apprentice Wizard is correctly listed as a CR 1/4 creature.
For further confirmation of this, the D&D Monster Lists PDF (as published in November 2017) has the Apprentice Wizard correctly listed as a CR 1/4 creature, attributed to Volo's Guide to Monsters. It also does not have a CR 1/8 version listed anywhere.