CVX does not allow division of variables (except in limited circumstances in gp mode).
Presuming eta is constrained to >= 0, you can move the denominator in the constraint to the right hand side. Then the objective can be dealt with by linear fractional programming, per section 4.3.2 “Linear-fractional programming” in https://web.stanford.edu/~boyd/cvxbook/bv_cvxbook.pdf
For future difficulties, you should show us the exact error messages you received.