Please change your iTunes/Playstore settings such that the location is set to Singapore.
You may follow the steps below to make changes.
If you are using IOS:
Open the Settings app.
Tap your name, then tap iTunes & App Store.
Tap your Apple ID, then tap View Apple ID. ...
Tap "Change Country or Region." If you don't see "Change Country or Region," contact Apple Support.
If you are using Andriod system:
On your Android phone or tablet, open the Google Play Store app.
Tap Menu. Account.
Under "Country and profiles," find your name and country.
If you don't have a payment method from the new country, follow the onscreen instructions to add a payment method. ...
The Google Play Store automatically changes to the new country.
After you have downloaded the Motorist app:
On the Motorist app, under the 'Dashboard' and 'Drive' section, click on the Esso logo icon, you would then be able to see the "Redeem Now'' and "View Vouchers'' which you can click on to redeem your petrol vouchers :)