Can I use Junk Arm if there aren't any trainer cards already in the Discard Pile?
No, you cannot play Junk Arm unless there is at least one Trainer card other than Junk Arm already in the Discard Pile beforehand.
Source: PUI Rules Team (2010-12-09), PUI Rules Team (2004-06-16)

Relevant cards

Note: All applicable cards may not be listed below.
HGSS4_87