This can happen when using the Steady BEM (whch is also called in the first step of the Unsteady BEM) or when using the DMST algorithm for aerodynamic loads on VAWT. In these algorithms, the code tries to find the induced velocity through an iterative process, that generally converges towards a final value.

In some cases, in particular when the conditions are not representative of a physical environment, it is possible that the algorithm does not converge, which leads to this error message.

It is unfortunately quite difficult to find what exactly makes the algorithm not converge, however you can try to
- check the values for wind speed
- vary the initial RPM
- vary the time step