iTunes 12.8 cannot connect to iPhone

iOS 12 requires iTunes 12.8 or better, running on OS X 10.11.4 or better. Looking at the compatibility requirements shown in How to upgrade to OS X El Capitan - Apple Support you should be able to update your Mac.

tt2

You wrote:

my MacBook Pro is from 2011

From: How to upgrade to OS X El Capitan - Apple Support

You can upgrade to OS X El Capitan from OS X Snow Leopard or later on any of the following Mac models. Your Mac also needs at least 2GB of memory and 8.8GB of available storage space.

MacBook introduced in late 2008 or later
MacBook Air introduced in late 2008 or later
MacBook Pro introduced in mid 2007 or later
Mac mini introduced in early 2009 or later
iMac introduced in mid 2007 or later
Mac Pro introduced in early 2008 or later
Xserve models introduced in early 2009

To find your Mac model, memory, storage space, and macOS version, choose About This Mac from the Apple () menu. If your Mac isn't compatible with OS X El Capitan, the installer will let you know.

Which suggests that you should be able to upgrade.

tt2

That's annoying. Any indication as to why? Have you looked up your model number at say EveryMac.com to see what I thinks you should be able to update to.

tt2