Hello Aldrin
You can't give thickness option since surface you have created has having blend surface some times these surfaces can't be offset upto the requirement, so use offset operation as per required thickness ( assume off set surface as B surface) and then connect two surfaces one is offset surface and another is your designed surface using sweep or blend option
And then use close volume command to create sold body
you could see if you could combine the surfaces into one solid and then shell to the thickness you need...
Might be worth a try
Sorry I saw it too late. Did you solve your problem?
I can help you with Catia V5.