To use the Nintex status column to filter SharePoint views and/or do operations on list items in Nintex you need to use the following status codes (instead of the actual status text):
NotStarted = 0
FailedOnStart = 1
InProgress = 2
ErrorOccurred = 3
StoppedByUser = 4
Completed = 5
FailedOnStartRetrying = 6
ErrorOccurredRetrying = 7