Why is so complicated to apple support to update iOS on my tablet

It's a matter of hardware incompatibility, since an iPad 2 cannot support iOS versions beyond 9.3.5-6, sorry.

Regards

Giulio

iPad2 cannot be updated beyond iOS 9.3.5 (WiFi Only models) or iOS 9.3.6 (WiFi & Cellular).

Apple ended update support for these devices in September 2016. The iPad2 cannot can be updated to iOS 10 or later versions, as the internal hardware does not meet the minimum technical requirements for new versions of iOS/iPadOS. The 32-bit hardware upon which the iPad2 is built is obsolete; the iPad2 is simply incompatible with more recent versions of iOS/iPadOS and Apps designed for 64-bit hardware.

Many App Developers have dropped support for older devices and iOS versions preceding iOS11/12/13 - and have withdrawn older versions of their Apps from the Apple App Store, now limiting utility and usefulness of older devices. 

If Apps that you need to use now have higher minimum iOS-version requirements, your only option is to replace your iPad with newer model. Even the basic current iPad7 model supports current iPadOS 13.x - and has hardware specifications that vastly outperform your old iPad

I hope this information is helpful - if only in clearly stating the limitations of your iPad2 and your potential options.