Jason,
You can reduce the FOS so that sliding and overturning checks become inconsequential as you mentioned. Generally you would have some service cases which would be used for bearing and stability checks and some ultimate cases which would be used for strength design. Having all load cases as ultimate would lead to the error you mentioned because in that case, the software would not have any loads based on which it can determine the foundation size.
You can reduce the FOS so that sliding and overturning checks become inconsequential as you mentioned. Generally you would have some service cases which would be used for bearing and stability checks and some ultimate cases which would be used for strength design. Having all load cases as ultimate would lead to the error you mentioned because in that case, the software would not have any loads based on which it can determine the foundation size.