You are missing your template file which is referenced in
Look at the code of the module-offline-payments like you described. They are all using a template from module-offline-payments/web/template/payment/...
so you would need to have the same path replicated in your module at https://github.com/techbertu/Offline_Payment/tree/master/view/frontend/web
This template files are responsible for displaying the content of the payment method if you select the payment method in the checkout.
F.e. the template for the banktransfer method located in:
is responsible for rendering the instructions by:
<p data-bind="html: getInstructions()"></p>
*edit for comment:
I guess the remaining problem is that the value in
window.checkoutConfig.payment.instructions[this.item.method]; for your payment method is missing. Maybe try this out by testing the
getInstructions method of your method-renderer just with a hardcoded string to be sure about that.
But what you will need to do to fix this is adding configProviders to provide the new config to
Please reference to
vendor/magento/module-offline-payments/etc/frontend/di.xml where the config providers are added for the module-offline-payments.