MacBook Air

I have a 2013 MacBook Air perfect condition. I have updated it and reset it but it won’t update past Mountain Lion OS X. ITunes is also on 11.0.4 but will not update to 12.4.3 which it gives me option too but doesn’t update. Error 102 pops up.

I have tried going to apples website to update past mountain lion to high Sierra and more but none work.

It won’t update anything and I have also already downloaded each download going down the list. ITunes 12.4.3 is the only download available. Please Help