It appears that not all primary resources on the URL in question are fetched using HTTP/2.
Why is this important?
Conceived in 1997, HTTP/1.1 no longer matches the current pace of the web. In contrast, HTTP/2, unveiled in 2015, is designed for the modern internet, offering advancements such as streamlined binary headers, concurrent resource delivery via multiplexing, and server-initiated resource push. Despite its release, widespread browser support was its bottleneck until most recently.
Presently, it's endorsed as a standard practice for performance improvement, acutely facilitating quicker resource delivery while conserving bandwidth.
What does the Optimization check?
Any internal URL harboring resources not delivered via HTTP/2 will prompt this Optimization.
How do you resolve this issue?
To shift toward HTTP/2, developer involvement is often necessary. Confirm the following with your hosting service provider:
The capability of your web server to support HTTP/2
Appropriate server configuration for HTTP/2 implementation
Server type and specific settings will determine the complexity of the upgrade process.
Further Reading