1. Paytm Sandbox credential Visit paytm website
2. Paytm App checksum kit visit paytm github repositry
Download that code and go into ConfigChecksum.php file and change Merchant Key
You can in app => libs => PGSDK File.
Note
Please check a youtube video description for developing a app without error. It is mandatory after new file setup of Paytm.
Now you can easily run your project but if any case you have any issues you can ask to me.