Are you sure you clicked on 1500 one ? Need to be quick during seeing, confirming and final purchase. But at that moment we are unable to control and end up clicking whatever is at the top ( without properly seeing it). Nevertheless glad it worked. Better luck next time , at least now you know the trick. You might have to start at :57 or :58 depending on your app response time.
You should see an option to see FUEL STATIONS NEAR YOU. Click VIEW ALL, Identify one's with green comment ( Successful redemption in last 24 Hrs). You can go to one of those pumps, confirm before hand whether they do accept indian oil voucher ( Such pumps are sometimes hard to find, sometime they happily accept, prefer COCO pumps if available due to high chance of acceptance over there) , if they do they will need this voucher code and your phone number and they will enter this info on their POS machine. You will get whatsapp alert after redemption and you will also get Xtra Reward points if you have IndianOil Xtra membership ( 1 RP per 75 Rs, 1 RP = 0.3 RS so effectively 0.4% Rewards rate on top of others). This is valid for 7 days.
You got to have patience ,focus and speed to get higher value vouchers. BTW for you, next voucher will be available after 7 days ( 21 March 3 PM).