Steam – I get an error message every time I try to install Portal 2 via Steam

I recommend verifying the game cache.. Its possible you are missing files.

To verify cache, right click the steam game in the list, properties, Local files tab and find the verify cache button. Click it and wait till it finishes.

It will download any corrupted/missing files.

As for the other stuff, it was too long ago to remember.

I experienced exactly the same thing some time ago.

I have no exact explanation, but my bet is that it's something with steam servers. Either maintenance or some outage. So the only solution would be to just wait for server to go back to normal, unfortunately.

Writing to support might help(or at least give a bit more info) but they do not answer questions quickly and the problem would go away when they would answer, I guess...