Ugh, I just figured out my mistake. I did not edit [Submit Date] to reflect as [Target Date]. Once I corrected that, everything worked as expected:
IF ([Status of Problem Record]="Cancelled Record") THEN 0
ELSEIF ([Status of Problem Record]="Completed Record")THEN ([Last Completed Date]-[Target Date])
Sometimes you just need to "talk" it out so thanks to anyone that listed in the original post.