Hi Usha,
Let's see if I understand...
You have a form (form1) On form1 is a TextBox (let's call it txtLocationCode) where a location code is entered. This location code needs to be a new (unused) code. Also on form1 is a CommandButton with the following code:

vlocation = location_code
USE ("c:\mine\land.dbf")
IF vlocation != land.location_code
   DO FORM ("c:\mine\land.scx")
ELSE
   DO FORM ("c:\mine\errorcode.scx")
ENDIF
RELEASE thisform

Is that correct?

If so, what I need to know is this:
What is the ControlSource of txtLocationCode? (Pardon me if the following is obvious, but...) When you go to the PropertySheet and select txtLocationCode (or whatever you named the TextBox), what (exactly) is in the ControlSource entry? Is it location_code? If so, this MAY be the problem. If the textbox is being used for a lookup, it can't be bound to a table or view, it should be bound to a persistant variable, for example, a form property.

To do that, you would go to the Form menu, choose New Property, and add a property like LocationCode.
Then, you would change the ControlSource to ThisForm.LocationCode.
And you would change the CommandButton code to

vlocation = ThisForm.LocationCode
USE ("c:\mine\land.dbf")
if !seek(vlocation, "land", "loc_code")
   DO FORM ("c:\mine\land.scx")
ELSE
   DO FORM ("c:\mine\errorcode.scx")
ENDIF
RELEASE ThisForm

This ASSUMES that you have an index on location_code with a tag of loc_code in your land table.

HTH,