This may or may not be an issue on your side:
Make sure you are not using a proxy, VPN or any other kind of traffic redirection.
Inform the owner of the website that because of HSTS, you are not able to visit his website.
He needs to configure a trusted SSL certificate that is neither self-signed, nor expired (read as: You can't connect via plain HTTP for the next year, until the HSTS entry on your device expires. This implies that the site in question was not added to the HSTS preload lists of any browser vendor).
The error you presented implies an untrusted certificate chain, with the certificate used by the Certificate Authority not being trusted.
This could be because of a private certificate authority (often used for VPNs to revoke certificates) or an outdated system configuration (CA certs are updated regularly, CAs like Let's Encrypt are very young and switch their CA certificates/add new ones once they are approved).
If you are the web admin: Configure your Web Server to serve HTTPS requests with a valid certificate.