Modeling colebrook white equation

Hello all

I am trying to implement the colebrook white equation in GAMS to solve a gas network design problem. However, GAMS keeps complaining that the model is infeasible with no solutions. Anybody has experience solving this problem?

I suspect it is because of the complicated nature of the colebrook equation:

I attach my code for your reference.
pipeline1timeperiod.gms (9.63 KB)