CF will never be null, and I expect you have structural issue.  UDS maintenance has a syntax check to be sure.

Add MessageBox pop-ups (or use debug prints) to check your progress as you test the script, so you can ensure it's running, getting correct values from each field, and where in your conditional statement the logic flows.

The syntax check past. I used the Volume field on CI Item as CF (Cubic feet). The field need to manually enter. I want to caclulate the UDF fields H W D / 1728 and get the value enter to "Volumn automatically. See screenshot below. Thanks much if any one can help.

Did you add pop-ups to confirm you are getting values for H/W/D?

vivian kwok as Kevin M stated above you'll need to change this like slightly as because CF will likely come in as zero, so you're logic needs to change somewhat.  I think below is probably closer to what you want.

...
if (CF)=0 then 
    retVal = oBusObj.SetValue("Volume", (H*W*D)/1728)
end if

If the volume is being calculated, then why conditionalize based on the current value?

H = 0
W = 0
D = 0

retVal = oBusObj.GetValue("UDF_ID_H", H)
retVal = oBusObj.GetValue("UDF_ID_W", W)
retVal = oBusObj.GetValue("UDF_ID_D", D)
retVal = oBusObj.SetValue("Volume", (H*W*D)/1728)

There are environmental concerns too.  Allow External Access, UDF's set up as numeric... Pop-ups narrow down what the problem is.  The way I look at it, script logic comes after the basics are proven working.

Thank you Kevin, & Jepritch, how to narrow pop up condition in this. Sorry, not too expert on Vi script. Appreciate your help.

retVal = oSession.AsObject(oSession.UI).MessageBox("","Words:  " & sStringVar)

Put lines like the above in your script (and recompile) to check the values in your variables.  Any numeric values should be converted to string...

cStr(nNumberVar)

Doing this will let you know what is working and what isn't.  If your H / W / D values are zero, the other statements are not the problem.

Thank you Kevin for teaching this.

The final script works as below

H = 0
W = 0
D = 0
CF = 0

retVal = oBusObj.GetValue("UDF_ID_H", H)
retVal = oBusObj.GetValue("UDF_ID_W", W)
retVal = oBusObj.GetValue("UDF_ID_D", D)
retVal = oBusObj.GetValue("Volume", CF)

if (CF) = 0 then retVal = oBusObj.SetValue("Volume", H*W*D/1728)
if (H)=0 then retVal = oBusObj.SetValue("Volume", 0) end if