Well, first you need to check if you have any security update available in your app store update section.
if you still have a pending security update, you should download it directly and install it manually.
im not sure which country you are located, but in some regions the security update only shows when you are using a VPN.
So make sure you don't have any security update in your mac's App store.
Then do the thing with iTunes,( uninstall and re install) don't forget to restart your computer and empty your trash in each step.
And finally connect your phone without opening iTunes, it should ask you the trust and entrust question , and afterwards warn you about the new iTunes software update , make sure you do the update successfully, cuz sometime it will jump out of the process without succession( mine did at first)
and after wards open iTunes and it will show your phone.
its not that complicated, I'm not an expert and didn't want to mess with my computer but this was pretty easy and worked for me.
If you had any issues let me know, i will try my best to help you get over this issue cuz i know how painful it is not being able to sync your phone with iTunes...
Best of luck
Cheeeers