Proof of Play should be supported with the 1.8.12 CMS although I would recommend upgrading your CMS to 1.8.13, as this is the latest version of 1.8 CMS and also includes a fix for Proof of Play. Just in case, here are the links for the CMS upgrade files and guides you will need:
https://xibo.org.uk/docs/setup/upgrading-the-cms
Many Thanks.