Every time I try to print a single envelope, the message "the number must be between 1 and 31" pops up. I can't figure this out.

I was able to fix this without creating another profile.  Unfortunately you have to do this every single time you want to print an envelope.

Select Mailings then Select Labels (do not select Envelopes yet)

In the Print Dialog Box change the label setting to a Single Label

Change both the Row and Column to 1

Now you can select Envelopes,   type in the delivery address and print.

Although I'm sure there must be an easier solution, several online sources suggest that recreating the user profile solves the problem. One way of doing this (I assume) is given at

http://www.omnitechsupport.com/forum/topic/169/when-try-to-print-envelope-get-message-the-number-must-b/

Some previous replies say that this error can occur if you have added a header or footer to the Normal template, but other users report that this occurs in a "virgin" install of Word 2007.

Another suggestion that is really only a workaround is to use envelope templates rather than the Envelope dialog; for some examples, see http://www.gmayor.com/downloads.htm for some downloadable templates.

FWIW, the "between 1 and 31 (points)" measurement usually refers to the "distance from text" of a border or page border, and I believe this error message is a rather generic one that we have seen in the past in other unrelated situations as well.