If the adjustment is not well defined, for example if no station has been held fixed, then the error file will state that a singularity has been encountered. A singularity can also occur if there is a hanging line which does not have a distance and angle measurement (and vertical measurement in a 3d adjustment), or if there is any other configuration defect in the network. Look at the listing file for more information about the singularity. It will indicate at which station or parameter the singularity was detected.