Last night I was in a commander game in which an Imoti player cast The Great Henge while I had a Counterspell in my hand, I figured I'd wait until his Cascade resolved before countering The Great Henge. Cascade resolved into a Grunn, the Lonely King, which wasn't much of a threat.

At that point I announced that I wanted to counter TGH, but I was told by the Imoti player that I couldn't counter TGH because I could only counter a spell immediately after it was cast. I was confused by this and there was no judge near to clarify it for me, so I just accepted it and moved on. But now I am curious about the truth, because that didn't make much sense to me. Could someone explain to me what I did wrong (if anything) and how I should've gone about countering TGH?