The Lump Sum need analyser determines the additional lump sum cash inflow that would be needed at any given point in the timeline (lump sum timing event) in order to prevent any future shortfalls from occurring thereafter in the plan. If you have selected a lump sum timing event which occurs after the last shortfall in the plan, the simulation will return a need of £0, as future expenses after the selected event can be met from existing sources.
This simulation will also return a need of £0 if you select Mortality as the lump sum timing event. This is because the final person in the plan is assumed to have died at the start of that year; therefore there is no cash flow to show thereafter, either for the client or the client's spouse or partner, and therefore no need to be met.
Further reading:
Need Analyser, Lump Sum - About the lump sum need analysis
Simulation Error - An error or outsized result displays when running a need analyser