This error indicates that the certificate that MATLAB Web App Server is using for SSL/HTTPS is not trusted by the client browser. This typically occurs when using self-signed certificates, such as certificates generated following this MATLAB Answer, or when the common name on the certificate does not match the URL you are accessing. You can click past this warning using the "Advanced" options to access and use the MATLAB Web App Server regardless of the certificate trust.
If you would like to avoid this warning, you can use a certificate signed by a trusted certificate authority (CA). These can typically be purchased or generated using third-party tools such as Certbot. CA-signed certificates should be automatically trusted by most browsers. If your end users will be accessing web apps through a load balancer or custom URL, ensure that the certificate common name (CN) matches the URL used by end users.
Some organizations may also have internal certificate authorities that can generate certificates that are automatically trusted by all internal machines. If your end users are all internal to your organization and are accessing MATLAB Web App Server from browsers on their work/school machines, this may be an option as well.
