WhatsApp has not enabled this option for all the beta users as of now. It appears that this is a server-based feature which will be gradually available to all the users. As of now, WhatsApp beta v2.18.21 for iOS has the payments option enabled. For Android users, this version is v2.18.41. Though everyone might not get it just right now, you can check if the payments option is enabled for you or not.
One interesting thing to note is that this feature works only if the payments option is enabled for both the sender and the receiver. Putting it in simple words, the transaction will be successful if and only if payments option is available to both the transacting parties.
The transactions will be carried out through UPI, and you can access the feature by tapping on the attachment icon just like the way you access the gallery, location, and contacts among other things. Once WhatsApp Payments option is enabled for you, on clicking on it, there will be a list of banks. Once you select your bank, you will see the linked account which you have to verify further. If you have never used the UPI based transaction service before then, you will be asked to register, and after successful registration, you will proceed with the transactions.
WhatsApp Payments once rolled out to masses will eliminate the need for other UPI based transaction apps. As most of the transactions are carried out digitally these days WhatsApp Payments is going to be a huge success in India.