Sagepay recently changed their SSL and it has been causing issues. To resolve it they suggest pulling down the certificate in the browser and adding it to your root store. They don't give instructions on this and cannot clarify further. I've contacted my hosting company and they say it's something I need to deal with.
I'm hosting on Siteground running PHP 7.3 if that helps
Sagepay "solution": https://www.opayo.co.uk/support/support/system-monitor
Issue solved by deleting the cacert.pem file in the Guzzle directory in the vendor file and adding a new one