In the English language version of Office, the thing you are allowed to set
is called the "Subject". Maybe it is called something else in your version
or language.

The only thing that Mail Merge lets you do is set the email Subject to a
single field (column) in the Mail Merge data source. So you can set it to
<Surname>, but not "Hello, <Surname>!"

The easiest way to fix this is to create a new field (e.g. called
MailSubject" ) in your data source that contains the text you want, and use
that for the merge subject field.

Peter Jamieson