Symptoms
Unable to create package:
'[Process]' Process is referencing the '[Method]' method on the '[SmartObject]' SmartObject that could not be found.
'[Process]' Process is referencing the return type '[Return Type]' of the '[Method]' method on the '[SmartObject]' SmartObject that could not be found.
'[Process]' Process is referencing the '[SmartObject]' SmartObject that could not be found.
Diagnoses
[SmartObject] does not exist on the source environment
Went through the process using K2 Studio but there no calls to this missing SmartObject.
Opened the process using a text editor and saw references to the [SmartObject]. It was under
Resolution
Using K2 Studio, go to the Object Browser > Process/Activity Data > Item References. There should be an Item Reference having the same Display Name as the [SmartObject] but empty properties when expanded. Deleting this and redeploying the process allowed for it to be added in a package.