Iris, it is a very obscure or unclear meaning. We can refer to it as an ambiguous proverb.
Like many "wise" sayings, it is not entirely clear; but this goes a bit further than that.
I do not think it has a clear meaning.
First, it presumes that you Do---Not---Have the thing that you "really value".
Second, it does not tell us the Identity of the thing you "really value".
Third, it presumes that you "really value" what you do not have; but that does not really mean anything, because actually, it presumes nobody, anywhere, possesses anything that they "really value".
Fourth, it could be generally concluded, that people desire what they do not have,
but surely, somebody has something they desire.
Fifth. Let's examine an old proverb concerning "possession".
"A Bird In The Hand Is Worth Two In The Bush".
or "Crap in one hand and THINK in the other, and see which fills up first."
It is an unfortunate fact, that there is a part of mankind, who enjoy appearing wise, by publishing confusing and nonspecific statements. A person with a little wit, can compose an endless array of confused and generalized statements to puzzle other people with.
There can be wise sayings but there are many which are unclear. There is in fact, an entire genre of literature which uses "generalizations" endlessly. Everything written in the
"New Age" books can be said to be "generally" true. That is why there is no specific information in them.
I like for my proverbs to be substantial, useful in some practical way.
I think it is important to take a bit of care about what "old saws" we pick up to use on our journey through life.