Hi YJ
That RESP command is an Mechanical APDL command. It is not a WB Mechanical function/feature. You can look up that command in the Ansys MAPDL Help - there is an example on using it though it assumes that a time transient analysis has been done and uses the displacement time history of a node as the input.
There is a LF post on using this though it looks like the format got messed up somehow; there are "n" instead of new line character so it's all one big paragraph! Here is the link to that:
https://forum.ansys.com/forums/topic/how-to-create-a-shock-response-spectrum-from-time-history-data-with-apdl-commands/
Mike
You are navigating away from the AIS Discovery experience