SSRS - Make text bold based on conditional formatting

I am using a simple expression to make a few values in a field bold:

=IIF(Fields!header.Value="TOTAL GROUP","Bold","Normal")

However, this is not making the value bold. but in fact removing the value and renaming the rest to Normal. I am not expecting to see this. What am I doing wrong?

It sounds like you're putting the expression for the FontWeight in the Text Box's Value rather than the Font Weight or Style (depending on where you see it).

I usually use the FontWeight property in the Properties window.

You can also use the Text Box Properties by right-clicking on the text box and selecting Text Box Properties to get the Properties window.