HTTP/2 serves your page's resources faster and with less data moving over the wire.
How the Lighthouse HTTP/2 audit fails
Lighthouse lists all resources not served over HTTP/2:
Lighthouse gathers all resources requested by the page and checks the HTTP protocol version of each. There are some cases where non-HTTP/2 requests will be ignored in the audit results. See the implementation for more details.
How to pass this audit
Serve your resources over HTTP/2.
To learn how to enable HTTP/2 on your servers, see Setting up HTTP/2.