There are a few threads on that messaging.
viewtopic.php?t=28158&sid=16f2cdb32f66b ... 5a66e91092
Essentially that device is not running compatible Home software with your system.
Check the IP and find the device.
May be limit as to its end iOS version it can upgrade to, below OS18 incompatible with OS18 and above systems (yours and now everyone’s)
If it is an unused device for Home App - disable the Home App completely within it to remove the messaging.
Alternatively:
If you have just updated everything and the device is uptodate OS26 - then I would expect this would be a one off messaging - that will resolve when it recompletes its key exchange security.
Glenn
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