Q: the computer does not allow actualization to Itunes 12.3 version

If your Mac OS X version is 10.6.8 (as shown in your profile info), the latest version of iTunes (12.3.2) requires OS X 10.8.5 or later.  So an OS X upgrade is required, if you need to run iTunes 12.3.  Your specific MacBook Pro model determines supported OS X releases. 

If you select About This Mac from the Apple menu, and click More Info, the "Model Identifier" would help identify your MacBook Pro model.

You have a late 2011 MacBook Pro.

It can be upgraded to El Capitan.  El Capitan (OS X 10.10) will allow you to use the latest iTunes.

Here is how to upgrade OS X.

Upgrade to OS X El Capitan - Apple Support

These are the MacBook Pro models with that identifier

http://www.everymac.com/ultimate-mac-lookup/?search_keywords=MacBookPro8,1

They can be upgraded to the latest OS X release, El Capitan (10.11).  The upgrade is free, and available through the Mac App Store.

You are making a large jump from 10.6 to 11.11; be sure to make a backup of your data.  Also, Snow Leopard (10.6) has a built-in "translator" (called Rosetta) that lets you run older apps that are written for PowerPC Macs.  OS X after Snow Leopard do not have this capability.  So, if you use any old Power PC apps (that are not "universal"), you can no longer run them with El Capitan.