offline installation

@Shwan Are you not using MDM for your Macs? (Looks like it from the image in your post) Apple docs for the com.apple.fileproviderd (https://developer.apple.com/documentation/devicemanagement/fileprovider) lists both "Allow Manual Install" and "Requires User-Approved MDM" as supported for macOS but those seem mutually exclusive. It's possible Apple changed the behavior for macOS 15 to disable manual install but failed to update the docs properly. 

We have some test computers (running OS 15 or later & beta OS) that cannot connect to the network, making registration impossible. Manual installation of offline files is required.

You'd need to contact Apple to get the definitive answer on if the requirements for the com.apple.fileproviderd payload were required to changed to require MDM deployment in macOS Sequoia. That said, if your environment relies on being able to manually install a .mobileconfig be aware that Apple has changed other payloads to require MDM installation in the past, and as MDM profiles are replaced by Declarative Device Management deployed configurations there will isn't an option for manual installation of those.

A lot of Configuration Profile payloads must be installed by MDM, it is reasonable to assume Apple has added more to this list. As @sdagley  said, you need to contact Apple.

Manually installing these profiles "offline" and not deploying them from a MDM is not 1:1 testing of how things will behave in your production environment and is not valid testing attestations. 

I think I’ve identified the cause of the error. By opening the profile downloaded from Jamf with iMazing and removing the ​File Provider​ payload, the installation works perfectly now. Thank you all for your help—I really appreciate it! 🙌