Hi Ganapathi,
During approval process if there is no change in the UI, and if you click on submit button then it will not do any validation check. If you try to select either reject or approve ( radiobuttons) then it adjusts the screen( which means that UI is changed, then you can see the error).
Please put a break point in below method
class : /SAPSRM/CL_CH_WD_BOM_SC
method : /SAPSRM/IF_CLL_BOM_SC~submit
This may help you..
Regards,
Krishna K