Unable to Verify $_POST Vars Error
There are two main things to try when fixing this error:
This script will help spot some known server issues that would cause you this error. Knowledge Base » s2Member Server Scanner
Follow the instructions in that article to spot these issues, and follow the advice given to solve each until you get an all-green report.
That setting would cause this error if it’s missing or incorrect. Please verify that your PDT configuration is correct. Dashboard -› s2Member -› PayPal Options -› PayPal PDT Integration
After fixing each one, do a new test purchase to see if the error is gone.
If you continue having this issue, try these other tips to help solve the issue: Knowledge Base » Common Troubleshooting Tips