We have resolved the issue of having invalid checksum during payment using Paytm payment gateway.
This issue is completely related to your API credentials that you put under your env file. In my case, I am giving the wrong paytm_merchant key.
So you just have to recheck your credentials provided by Paytm. To get your API credentials you have to visit the following link https://dashboard.paytm.com/
In the sidebar under the Developers section, you can find the API keys as well as the API documentation.
I hope this post helps you to resolve your issue if your issue doesn’t resolve feel free to comment. We are here to help you.