It is relatively well known that hieroglyphs were not fully translated, or at all for that matter, until a while after the Rosetta Stone was found.
It is less well known that the script known as Linear A (https://en.wikipedia.org/wiki/Linear_A) still hasn't had any of its texts deciphered.
Suppose I have an ancient civilization or something. Now, what (fictional) ancient civilization doesn't have ancient and powerful artefacts? Now, these guys are smart. They know better than to leave a potentially world-ending device behind without an instruction manual including a whole lot of "Don't do this, or this island will sink" and similar warnings.
However, because of dramatic necessity, I need absolutely no one to be able to read this manual. Otherwise, they would know better than to blindly try pressing buttons to see what happens.
To make this more challenging for you, the civilization would probably at least try to give a few hints on how to read the script, but if this makes it impossible, you can assume the dictionary has been hidden too well, or is eroded, or something.
Otherwise, the dictionary does exist in some form. (This is why Etruscan is not a good answer, because they would probably written their "Rosetta stone" in proto-latin, and that would probably make it easy to understand).
some extra context
The device these instructions refer to is basically a universal property (aka, reality) editor. Most of the available vocabulary would be used somewhere.
On the topic of vocabulary, let us say that it is a language of roughly the complexity of Latin, with a vocabulary the size of English (for simplicity's (as in simpler for you who are answering, not the people who are trying to read this) sake).
What would a helpful civilizations's script need to be like, if no-one from modern times can read it?
I'm sorry, there are just so many wonderful answers. If you are interested in the topic, I definitely recommend you read them all. Not just the one I (will eventually, after much deliberation) accept.
In the comments, someone inevitably mentioned the Star trek tng episode where the universal translator fails, because it doesn't get the literary references. I'd assume that these guys would be smart enough not to write in this way without leaving a record of what they are referring to.