Hi!
You need to pick a weight that the font supports. If you are talking about the Google font Merriweather, it doesn’t have a 600 weight.
If you try to use an unsupported weight, the browser will use a fallback weight.
[Edit] Not that you should pick a font based on the number of weights it has, but Google Fonts has a filter to help find those that support many styles and weights. Sarabun, for instance, is one of the nicer ones that supports all weights between 100 and 800.
