Drupal webform billing fails: State/Province field is required
Singapore does not have States/Provinces but there are Regions. But these have not made the list. You can add the regions to the civicrm_state_province table via the API Explorer

In Civi go to Support > Developer > API Explorer

In Entity select StateProvince 
In Action select Create
For Parameters:
     State = Central Region
     State Abbreviation = CR
     Country = Singapore

Scroll down and you will see an Execute button. Click that and confirm Write to Database

You can repeat this for each Region or you can do as Karin suggested and just create one state "Singapore"