Go to table V_T588M and select the MP000800 and enter
then you have to select the Variable key of your country grouping.
for instance if i am changing it for India which is country grouping 40 i will go to variable key 40 and double click and make the PREAS as Required field.
if there is no variable key for your country grouping then select the line item that does not have any variable key and make the changes , the changes will reflect in your infotype 0008.
pleaes look at the screen shot for your reference. this is for India so i used variable key 40 here.
you might not saved the table entries after changes. please do the changes in T588M table and then save it. and then try
one more reason, if you changed the P0008 feature then you need to activate it. then try.
if also if it is not reflecting then go to main screen of 0008 infotype which is without variable key , do the changes here and try.