Dear Ahmed,
The error regarding Rzero that you are reporting is generated by AeroDyn if a blade node ever extends beyond the radius of the blade tip as defined at model initialization. My understanding is that the error can be generated from some combination of precone, small element length (DR) at the tip, or excessive blade stretching.
If excessive blade stretching is part of the problem, you are likely also getting a warning from FAST regarding a small angle assumption violation. We’ve answered a question regarding how to address this warning in the post dated October 1, 2009 in the topic found here: NREL 5MW controls (DLL interface).
If excessive blade stretching is not part of the problem, a suitable workaround may be to increase the length (DR) of the outermost element (unless you have to increase DR so much that the discretization is too coarse for sufficient accuracy).
I hope that helps.
Best regards,