for loop in sdc file

SDC is just tcl with a different extension so yes, you can use loops. But they need to be correct tcl syntax.

In your code, tcl has no way to tell that you want to replace $x instead of $x_DD. I don't have access to Quartus to try but I believe you need to use ${x}_DD. If that does not work, simply use string manipulation to build the signal name before using it:

Set signal "ATA"

String append $signal $x

String append "_DD"

Or something like that.

Hope this helps

DK