advanced variables issue

It is in your PayPal account when you create a button. Are you looking in your PayPal account?

Dear, I have followed the following document:
https://simple-membership-plugin.com/wp-content/uploads/documentation/simple-membership/membership-basic-setup-and-configuration.pdf

but there is no create button area in payment setting or other settings as you mentioned in the pdf file.
for example i have created this variable:

notify_url=https://localhost/psnx/?swpm_process_ipn=1
custom=subsc_ref=3
please tell where can I put this code in order to proceed?

Thanks

ok I have got it. When I try to generate button there is following error in paypal site:

username password creation:You can not use this username and password generation feature because you have not yet activated IPN. To use this feature, you must read the password generation manual and follow the specific setup instructions.

can you tell me how to activate IPN.

Thanks

The following page has a video that should show you how to enable IPN:
https://www.tipsandtricks-hq.com/ecommerce/how-to-setup-paypal-ipn-notification-2796