I successfully installed GTalk Video App after restarting 3 times.

Third time, I went directly to store and installed it. No errors.

Once I added a new account for my GMail logged in and available online, I am able to see the option for video call under my GMail contacts.

So, I guess this has something to do with some cache file getting locked and probably some background process is putting a lock on it.

Edit:
I checked the application update tab and it said it is up-to-date (bug 1044 gone). I clicked application update refresh button and it took a long time. I closed the app by swipe down.

I went back to the store and tried to install another app. I got the same error.

So, I guess the application update app is locking something and causing this issue.