when i simulate the liquid-solid two-phase flow, the warning is:

warning 16 ------ void fractionof cell 22854 is too large - 28.1 it is set to - 0.4

what does it mean? how should i correct it?

In simulation with lagrangian two phase, it's assumed that you have a maximum void fraction of 40% (0.4). If you exceed this value, you have either too small cells or to much of the second phase in part of you model. If you get 28.1 there must be something very wrong with your set-up.