If a area has a workbench is the area scrappable and can you build a settlement?

yes

If you find the RED workbench, then yes. If you find a random armor/weapon/chemistry/cooking crafting station, then no. You can use them for modding your gear or crafting various items at the chemistry and cooking stations, if you happen to have the appropriate materials with you.

The actual settlements will have a red workbench just like the one in Sanctuary.

Thanks because I just killed everyone in the Atom area but can't scrap or build.

The Crater of Atom? That's not a location you can build in. There are some various crafting stations around, but not a workshop bench.

And some areas might not let you use it until you pass a requeriment, normally related to a quest.