In reply to Nimisha Varadkar:
Please use code tags making your code easier to read. I have added them for you.
You are attempting to put a procedural NBA statement inside a generate loop. The code inside a generate-for loop is at the module level unless you put an initial always block inside it. What you probably want to do is:
for(genvar j =0; j<32; j=j+1)
begin
let temp = {6{data >> (j*6)}};
assert property (data_valid(temp));
end
Also, the expression
temp >= -'d32 and temp <= 'd32 will not work unless temp is a signed variable type.