"Connection not secure, Certificate signed by untrusted issuer" being displayed to certain users

Hii,

The user is using Google Chrome Version 103.0.506 in a 2008 iMac with OS X El Capitan.
I see that the certificate is not compatible with these user specifications.

I’m assuming that to fix this I would have to upgrade the plan and add my own certificate?
Also for the school network problem blocking the site, is this a common problem for Vercel deployments?