Lillian,
 Try....
Dim strTemp as Variant
StrTemp = acViewPreview
(Tested OK)

But... I 'm not sure why your using a variable for acViewPreview, when a
simple IF statement (using acViewPreview) would do the trick just as well...
with no variable needed.

If RetValue = 6 Then
   DoCmd.OpenReport "rptEndOfMonthALL", acViewPreview
Else
   DoCmd.OpenReport "rptEndOfMonthALL", acViewNormal
End if

Why use a variable to represent only 1 value,,, just use the value itself.
It's a bit like saying 1=1.

Also, I think my code is much easier to read, and understand...
Your call though...

Hi Lillian,

The problem is that acViewNormal is not a string value; it is a member of
the AcView class - essentially a built-in constant.  You could declare it as
Long, and your code will work (if you remove "Set " from your variable
assignment).  However, I'd suggest simplifying your code even more, as
follows (and using the defined constant vbYes to check the return value of
your MsgBox):

 If MsgBox("Preview Reports that have been checked?   Click Yes to Preview
or No to Print", vbYesNo) = vbYes Then
   DoCmd.OpenReport "rptEndOfMonthALL", acViewPreview
 Else
   DoCmd.OpenReport "rptEndOfMonthALL", acViewNormal
 End If

HTH,

Rob