can't back up iPhone to my MacBook

No sweat, mate. You need a fresh backup to make sure you don't lose everything if something blows up and trying to resolve the iTunes backup problem could blow everything up.

Just shoot an iCloud backup and now the heat is off and you can breathe easy and take your time and relax. Then, by and by, state exactly what fault alerts are coming up when you try to connect the iPhone to iTunes on your MacBook.

What issues do you have or what error?

Have you tried this? https://support.apple.com/en-us/HT203977