HI Mahesh,
Welcome to WFA! In the parameters of a command you can hard code values but you have to make sure that the parameter type is suitable for something hard coded.
In create volume for example you see that the aggregate parameter requires a reference to an aggregate (in the yellow tooltip and in the green (R) in front of the parameter name):

So for this aggregate parameter you have to provide a reference to an object that you found using a finder (either here, or somewhere earlier in your workflow). You can however put hardcoded values in the finder. So for your workflow you probably need to click click the little [...] button in the aggregate parameter field and then select some criteria (which can be hard coded) to find a reference to an aggregate object. When you hard code values enclose them in single quotes.
Also check the developer guide that is part of the WFA 2.1 documentation set for more info on how WFA works.
Hope this helps!
Chris Madden
Storage Architect, NetApp EMEA