https://wccftech.com/apple-fixes-studio-display-update-issue-addresses-code-signing-issue/

It was recently reported that some Studio Display users were unable to update their displays to the latest firmware. Apple took note of the issue and it has now issued a fix for the problem. As it turns out, the issue stemmed from the software being unverified by Apple's servers. Scroll down to read more details on the subject.

It was previously reported on Twitter that the latest firmware build for the Studio Display was not signed by Apple since last week (via MacRumors). This disallowed users from updating their Studio Display software to the latest version. Once the issue was discovered, Apple resigned iOS 15.4 software update, allowing users to update to the latest iOS 15.4 version.