Hi there. Sorry you’ve run into this issue.
This is similar to the “security header is invalid” error, and there are some troubleshooting tips here: https://www.paidmembershipspro.com/security-header-is-not-valid/
From that post, here are some things to try:
* Make sure you’ve selected the correct Gateway Environment (Sandbox/Testing or Live).
* Make sure you are using the correct Account Email, API Username, API Password, and API signature for the environment you selected. (Your live and test API information will be different.)
* Make sure that your web server is running up to date SSL/TLS software.
* Make sure you have a Site Title set in your WP dashboard General Settings.
* Try removing any special characters from your membership level names.
Let me know if any of this fixes things for you.