If you can't validate the purchase code, please ensure the following:
- Your server must use one of the supported platforms listed here: https://letsencrypt.org/docs/certificate-compatibility/#platforms-that-trust-isrg-root-x1
- If you use OpenSSL, please update it to version 1.1.0 or newer. Version 1.1.0 was released in 2016 and is the oldest version that supports the new LetsEncrypt certificate chain. However, we would strongly advise you to only use the most recent OpenSSL version (3.0.0 as of September 2021).
- If you must use an older OpenSSL version, please refer to this guide: https://www.openssl.org/blog/blog/2021/09/13/LetsEncryptRootCertExpire/
- If the above doesn't resolve the issue, put the attached file in wp-includes/certificates/ca-bundle.crt