The sentence "If you don’t watch till the end, you’ll miss the treasure." is grammatically correct. However, the phrase "Or until the end" is a bit unclear in its context. If you intend to offer an alternative phrasing, you might say, "Or if you don't watch until the end."
So, a more polished version could be: "If you don’t watch till the end, you’ll miss the treasure, or you might say, ‘until the end.’" This way, it clearly