Further clarifications:

For those suggesting solutions, the following are NOT factors:
*The document is not opened (Readonly).
*We have write access to the shared drive where the document resides,
and to the document
*The creator of the document has not prevented (at least knowingly) or
restricted access in any way.
*The fact that the document was not created by the person attempting to
turn on sharing is not a factor. This is not a restriction on other
documents not created by the person invoke sharing.

It appears that there is some feature used in this spreadsheet which
then renders it unsharable, so the "Share Workbook" feature is turned
off and not available to be selected. Otherwise, we are doing nothing
different from other previously shared workbooks of a similar nature.
The only factors different in my case is that there are some more
complex features in use (Lists, functions, etc.) than in some other
shared workbooks.

Any solution to this question would be most appreciated!