In order to close an account, the user cannot be an Admin on any Listings. In order to achieve this, Admin access should be transferred to another account.
Step 1A: Ensure Admin access for all associated Listings is applied to another account if that is not already the case. If the account being closed was the only one eligible, you will have to identify and confirm another suitable candidate.
Step 1B: Remove Account from Listing | Transfer all Listings from on Account to another | Transfer a Single Listing
Step 2: Once Step 2 is done, the 'CLOSE ACCOUNT' button should become active. Select it to close the account.
Note: Add a note to the account detailing why you are closing the account and who you transferred the listing to.