If you insert the fields
{ MERGEFIELD Total_NB_Premium_away_from_Plan }
and
{ MERGEFIELD Total_Premium_of_Team_Sales_Target }
directly in your mail merge main document, outside an IF field, not nested in an { = } field, with no formatting switches, what results do you see?
Word will only apply a numeric format to things that it decides are numeric (broadly speaking, that includes numeric and currency amounts that it can recognise). It will leave anything else unformatted. So if you have something that looks a bit like a number
like "$-164,209.00", the chances are that Word will leave it exactly as it is.
When Word does not recognise something as being a number or a valid numeric expression, it will generally display one error or another when you try to put it in an { = } field. So, e.g.
{ =$-164,209.00 } would give you the syntax error that you are seeing. (Assuming "$" is set up as the currency symbol in your COntrol Panel's regional options. If the regional currency symbol was , you would see a different syntax error.)
The problem in this case probably originates in something that has happened in Excel. Normally, things that are numeric/currency in Excel come through as things that Word treats as numeric. But if for example, someone has entered a number formatted as text
in a column that has texts rather than numbers above it, Word may receive something that it does not think is numeric. You may have to fix your sheet before Word will "see" the numbers correctly.
When you have got to the point where WOrd sees the numbers correctly, you should be able to use
{ MERGEFIELD
Total_NB_Premium_away_from_Plan} \#$,0.00 }
(and the negative numbering formats if you want) without nesting the { MERGEFIELD } inside an { = }