Hi S_Shipper,
Based off what you provided in your post, it is a little difficult for us to pinpoint the issue here. It sounds like this error is likely occuring from some customization done on this screen, so I can't say for sure where this is coming from exactly. My advice would be to first figure out where this error message you are getting is being generated from. The error message suggests to look at the Action Sequence #5, I am not sure which button this would be linked to, but I would suggest checking your "save" button first, and if you have a "Retreive Value from Table" action as #5 in your list, this would be where the problem stems from. I would also have to guess from this error message that you are trying to retreive a value from a table, but the table has a blank/no value for this field, meaning it can't retreive any data. You might have to eliminate this action, or see why you are retreiving a value, if there should be one.
Hopefully my suggestions above might give you some further insight into how to resolve your issue.
-Ben