For the segment value to be available to select, the full account string that includes that value in the segment's place in the string must first be added in General Ledger setup, Define GL accounts.

If the account exists yet the segment value is not in the drop-down, confirm that the digits are in the correct place in the account and/or refresh the browser.

Note: segment values are sourced from the account number field on the GL account record, NOT the Alias.