If we check the standard open problems in Mathematics which they are understandable even for a student in Midlle school as example if we take this problem : " Is there an odd perfect number" ? it's understandable at a least for high school level or middle school but it's very hard for solving .
My logic basic is :any unsolvable problem never be understandable at all.
My question here is: Why an understandable open problem In mathematics can not solvable ?.
Note: I meant by understandable problem " popular problem "
Thank you for any help
Since you are not giving a formal definition of "not solvable", let me give you an historical counterexample to your claim.
The quest for exact algebraic expressions giving the roots of polynomials has been a central topic in mathematics for centuries. However, as a consequence of the work of Abel, Ruffini and Galois, the solutions of the equation $x^{5}=x+1$ cannot be expressed in terms of radicals. This is a very simple example of a problem that is not solvable (by radicals).