"Changelist may not be deployed while a deployment is already in progress" While Enabling State and Country/Territory Picklists

This scenario is resolved by going back to step one, inactivating all picklist data temporarily, enabling the feature in this empty state, and then starting from step 1 again. To resolve:

 1. Click the setup gear in the upper right corner and navigate to setup
 2. In the quick find, type 'picklist' and select "State and Country/Territory Picklists"
 3. Click the link in step 1 called "Configure States, Countries, and Territories"
 4. On this screen, at the top of the "Active" column, click the checkbox to select ALL Countries, then click it again to deselect ALL Countries
 5. Click Save
 6. Back on the State and Country/Territory Picklists setup page, proceed directly to step 4 and click "Enable." This will enable the feature with all inactive values.
 7. Proceed back to step 1 on the State and Country/Territory Picklists page, and follow each step (configure, scan, convert, enable) normally.