It is not Regular, this comes due to (y$z^{n}$)${^k}$
Regular Language/ Finite automata doesnt have memory.
The language is to accept y$z^{n}$ ‘k’ times.
i.e. if n = 2 and k = 3, yzz yzz yzz all three times. The automata has to REMEMBER y$z^{n}$ and everytime repeat only that, i.e whatever the first time value of n was, the automata has to accept ‘k’ times only that value of n.
Going by the NFA you made, if I want n=2,k=3 (m=X): $x^{X}$yzzyzzyzz, Then your NFA can also accept $x^{X}$yzzzzzzyzyzzzzz (fails to regulate the value of z) Hence a memory/stack is needed.
Language is atleast CFL
