Try resetting the Sena device using the instructions in the device user guide. Then go to your phone, if the Sena appears there, remove it, turn off Bluetooth, then go to Settings>General>Reset>Reset network Settings. Understand that will also reset your wi-fi passwords. After doing that, put the Sena into discovery/pairing mode. then turn on Bluetooth on the phone and see if it will pair then.
After seeing this image that you just posted, i suggest you contact Sena and see if their device supports the iPhone 8 and the latest iOS and Bluetooth stack. They should be able to answer that question.
i-phone bluetooth connection with Sena 20s
