Sequential shock test

Hi,
I performing a modal transient analysis.
My concern is , the part shall be subjected to a half-sine shock of peak acceleration of 50g and pulse duration of 8 milliseconds.
For both the tested-device shall be subjected to three shocks in positive x axis and 3 shocks in Negative Y axis in all three directions.
Total of 18 shocks.
Usually in lab only one part should be tested for all 18 shocks
I have created proper load step for modal transient analysis
for second load step i have called 1st load step by using STATUS(PRELOAD) and so on
I am facing this error.
*** ERROR # 1999 *** in the input data:
In SUBCASE 2: card STATSUB(PRELOAD) references wrong type SUBCASE 1.
*** ERROR # 1999 *** in the input data:
In SUBCASE 3: card STATSUB(PRELOAD) references wrong type SUBCASE 2.
*** ERROR # 2360 ***
STATSUB(PRELOAD) cannot be nested: SUBCASE 3 PRELOAD from SUBCASE 2.