Cannot enable iTunes wifi sync

Did you try syncing with the cable first? If not you might want to try that. Sync with the cable. When the sync completes, select the option to sync over WiFi. Quit iTunes, restart your computer and phone and then try again. This shouldn’t matter, but it might help if the phone is plugged into the wall socket to charge as well, at least for the first time.

Another thought.... do you have iCloud backups enabled? I had problems when I had iCloud backups enabled on my iPhone 5s and iPad Air 2 unless I turned off iCloud backups on both devices first. I never sync with the cable anymore and as long as I have the iCloud backups turned off before I sync, I can initiate the sync from the device or from my Mac.

You said in your first post that it was iffy to begin with. I have found a number of Apple services that are just supposed to work to be iffy at times. It may just start working for you for no reason at all, other than it should. I hope that it does.

I don't have a fix for you, just some more observations:

Trying to sync iOS 11.2 on iPhone 6s and iPad Air (series 1). To MBP 13" late 2016 (4xUSB-C +Touch bar - macOS 10.13.2)

Sync via lighting to USB-C cable works OK

Sync via wi-fi works if initiated by applying power to either iPhone/iPad or from sync button in iTunes on the macbook.

On iPad/iPhone - wi-fi sync settings page does not acknowledge connection to MBP.

On iPhone it says connect to iTunes via cable and select the sync over wifi checkbox in iTunes - That box is checked.

For both iPhone and iPad, unchecking/apply/check/apply/sync whilst tethered does not resolve. On iPad the wi-fi sync settings page, an old Win7 system is listed. Newer Windows 10 and the MBP are not - All have synced via cable and wifi to both devices in the past (but the iPhone 6s has been wiped/restored since connecting to the Windows systems). At that time the MBP would have been on Sierra (10.12), and iPad/iPhone on iOS10.3.3

Use reset iPhone/iPhone privacy settings to reset the trusts (and of course re-allowed the trusts).

So for me it's the settings page in iOS which is misbehaving and it could be iOS11 or High Sierra related.

Oh joy!