Q: Can not run Mavericks upgrade from the App store

I have an old iMac I am trying to update and I cannot get the Mavericks upgrade to run from the App Store. It is an iMac 7,1. I manually upgraded it from 10.5.? to 10.6.8. I then went to upgrades in the app store. Mavericks and El Capitan were listed. I tried both but got the same message:

"We could not complete your request. There was an error in the App Store. Please try again later (4)"

I find it hard to believe there is a problem with the app store so anyone have a clue what might be amiss?

thanks

john