I've experienced the same issue when trying schedule iOS 15.7 update for iPhone. I'm sending ScheduleOSUpdate mdm command without ProductKey but with ProductVersion value. In response I'm receiving empty UpdateResults. On device I see logs like below. Is scheduling iOS 15.7 using ProductVersion broken? iPhone dmd[440] <Notice>: Received request: <DMFScheduleOSUpdateRequest:0x12103bfa0 { Action : default Product Key : None Product Version : 15.7 Use Delay : NO }>, from client: <CATTaskSession: 0x11fe20a80 { state = Connected, session = B9BFD8D3-F7F5-43A9-86A9-8D031FF62F5C, transport = <CATXPCTransport: 0x12101bc10 { state = Connected }> }> iPhone dmd[477] <Notice>: schedule-os-update done, result = <DMFScheduleOSUpdateResultObject:0x10322f590 { Action : download Product Key : None Error : Error Domain=DeviceManagement.error Code=2213 "" UserInfo={NSDescription=, NSLocalizedDescription=No update available., NSUnderlyingError=0x10312f880 {Error Domain=com.apple.softwareupdateservices.errors Code=3 "(null)" UserInfo={NSUnderlyingError=0x10312d700 {Error Domain=SUCoreError Code=8406 "[kSUCoreErrorScanNoUpdateFound] no update found" UserInfo={NSLocalizedDescription=[kSUCoreErrorScanNoUpdateFound] no update found}}}}} }>

Thanks for your reply. Do You know can I install every update version which is included in gdmf.apple.com service? If not, is there any service, or any other place where I can find which update version I can schedule to install? We're trying to install updates version by version, not necessary always the newest one.