This is what happens when you use https for security but the certificate was generated on the server and not signed by anything further up the chain.
HTTPS security alone leaves you open to a man in the middle attack, which is sort of what the message is hinting at.
You could turn off https (therefore using http) which would suppress the message but means the content isn’t encrypted and could be sniffed by other machines on the same (wifi?) network, or you can just ignore it.