How to eliminate this quadratic forms?

I have a problem: Q(2,N) is a two dimensional vector, It is constrained by the following equations:
norm(Qn(:,x)-Qn(:,x-1)) == A, and produce this error :
Invalid constraint: {convex} == {real constant}
I wonder how to solve such a problem, I’ll appreciated if my problem is solved.

Nonlinear equality constraints are non-convex.

However, if the == is changed to <=, the constraint would be convex and would be accepted by CVX. Whether that modified (relaxed) constraint would be adequate for your purposes is another matter.