I had same problem.with Motorola Moto Z2 play with Android 7.1.1
I got Tomtom support by mail, but was useless, even they don't know about Bluetooth versions.
I found a process to get it work, hope works for most of you.
Try this solution (long process):
1. Connect the watch to the computer and erase all data (factory reset).
2. Register the watch and CHANGE the name. Use only one word, no special characters or spaces.
3. In the phone disconnect Bluetooth and Wifi. Then go to Configuration/Apps/TomTom Sports and Erase cache, the erase all app data. Don't uninstall.
4. Turn off your phone for 1 minute.
5. Turn on your phone and uninstall the TomTom Sports app.
6. Go to Configuration/Erase connection data. You have to look for this option, my phone is in Spanish and I don't know this option name in another language. WARNING: This option will erase all WiFi data and Bluetooth pairs. Yes... You will have to enter all WiFi passwords again and pair all your Bluetooth devices.
This process is important as this will prevent the same sync error happens with the same devices.
7. Turn off your phone (yes, again wait 1 minute).
8. Turn on your phone and Bluetooth (just Bluetooth, no wifi). Go to Bluetooth settings and change the phone name to one word, no spaces. This is optional, but recommended.
9. Install and register the app.
10. Now... Try to pair your watch with your phone following the TomTom instructions in this page.
11. Important: If you're success, Everytime you sync your watch, keep the phone screen on and the TomTom app.
To disconnect safety, in your watch go to Phone/Sync/ and turn it off. The you can turn off the App and Bluetooth on the phone.
To reconnect again, always star with phone app and Bluetooth, then with the watch Sync option.
Good luck to all of you. It's a problem that TomTom hasn't interest in solve.