What if an HTTP/1.1 client talk to an HTTP/2 only server and what if an HTTP/2 client talk to an HTTP/1.1 only server?

It's important to take in consideration that the most implementations of HTTP/2 uses it over TLS 1.2 with ALPN protocol (Application Layer Protocol Negotiation). Thus the client just start the standard TLS connection. As the part of such communication the client sends "Client Hello" to the server with some information:

It's like: "Hi, Tom! It's Bob. I speak German, Russian and English. Let's talk a little". And the server send "Server Hello":

"Hi, Bob! I suggest to speak German or English". Then the client send one more short message "OK, then let's speak German" and he start to speak German without waiting of any response from the server:

The whole communication looks like on the picture below

Because both the client and the server start the communication just using TLS 1.2, which the both know. They start the main communication after the protocol negotiation. Thus the problem which you describe could not exist in the practice.

If a browser only supports HTTP/1.1 and the server only supports HTTP/2, they cannot communicate. The server will not recognize what the client sends (in particular there will be no connection preface, which the server treats - following the specification - as a connection error), and will close the connection.

"A browser that only supports HTTP/2" does not exist; if they support HTTP/2, they also support HTTP/1.1. But let's assume that such browser exist.

In this latter case, the server will see the connection preface and will not recognize the PRI method. What exactly the server does in this case depends on the server. It may return a 400 Bad Request, or perhaps just close the connection, or it may trigger an internal server error.