Dear Pablo,
The error you receiving about “no valid value of phi” is a quite generic error meaning that the blade-element/momentum theory (BEMT) routines in AeroDyn can’t find a solution. Usually this error is indicative of a numerical problem with the simulation i.e. the error is usually preceded by a large jump in rotor speed or blade deflection. Is that happening in your case? If so, you’ll need to resolve the problem before proceeding. If not, you could see if an upgrade to FAST v8.15 (just released last week) fixes the problem: included in this update to FAST is some improvements to the AeroDyn BEMT solution procedure.
I’m not sure I understand why you would be choosing different blade-pitch angles below rated, but this may cause some problems i.e. the controller may not be initialized properly or it may be difficult (or take a long time) for the solution to reach a quasi-steady state. See the following forum topic for my recommendations on setting initial conditions for FAST simulations: Initial conditions for tower and rotor.
I hope that helps.
Best regards,