Clients that support HTTP/2 typically default to HTTP/1.1, and only use HTTP/2 if the server advertises support for it via the ALPN field during the TLS handshake.
Some misconfigured servers that do support HTTP/2 fail to advertise this, making it appear as though they only support HTTP/1.1. This can lead to people overlooking viable HTTP/2 attack surface and missing associated vulnerabilities, such as HTTP/2 downgrade-based request smuggling.
If you want to use HTTP/2, make sure the server is configured to advertise it correctly. Otherwise, consider fully disabling it server-side to reduce unnecessary attack surface.