You have 2 issues of syntax.
1. The For() loop requires the termination expression to be a comparison of some sort.
2. You have specified a column called "A" and a memory variable called "a". Because you did not show any scoping to indicate that the variable "a" is to be treated as a memory variable, JMP assumed you were referring to the column "A". Remember, JMP is not case specific.
Here is a corrected version of your script
db = Current Data Table();
temp = Column( "A" ) << Get Values;
temp2 = Column( "B" ) << Get Values;
Row = N Row( db );
::a = 0;
For( i = 1, i <= Row, i++,
::a = temp[i];
g = 0;
For( j = 1, j <= Row, j++,
g = temp2[j];
Col Minimum( a - g );
);
);
Jim