Ring Oscillator on FPGA for TRNG
This is not a ring oscillator, because r0 is unknown. You probably meant r_out:

and a(n6,r_out,start);


Also see: rout vs r_out.

If you correct your topology, you will run into another problem: a timing (delta) loop, basically a correct ring oscillator without delays will hang your simulator at 0 time. You will have to introduce a delay in your "mynot" macro to allow for exiting the delta steps from time 0.