in reservoir mass balance (bilan), i have told to make sure that the initial and end of period storages are equal.
is there any way that i can do it via automation, or it must be done manually?(actually i am using API and i cannot do it manually)

The only way to force WEAP to keep the reservoir storage constant (not changing) is to set the Top of Conservation equal to the Top of Inactive storage. This will prevent WEAP from releasing any water from storage, but also from storing any additional water.

On what principle water balance works in WEAP

thanks, but i mean first time step(initial like the first month in current scenario) and the last one("end of period" like end of reference scenario).

If you want the reservoir storage after the last timestep of the simulation to be the same as at the start of the simulation, you could set the Top of Conservation and Top of Inactive storage to the initial storage value.