I'm not sure if i'm not making the character right, or if its behind a paywall, or what. Someone told me to try out a fighter samurai  build but on the character builder i cant seem to be able to choose samurai.

In fact apparently champion is the only available archetype to choose from, why is that?

Well, that's what you get for free with the Basic Rules on DDB. All other subclasses are pay to play.

Samurai is in Xanathar's Guide to Everything. You need to purchase access to that content here on DDB to use it in the online tools.

To elaborate on what others have said above, the Samurai subclass for fighters was printed in Xanathar's Guide to Everything. Only one subclass for each class was printed in the freely available Basic Rules/SRD, all others you have to buy access to (yes, even the ones in the Player's Handbook).

You can either buy the whole book for ~29.99 USD (Note: the $19.99 compendium option is ebook only, it doesn't unlock content in the character builder). You can also buy the specific Samurai subclass piecemeal for $1.99. To do this, go the Xanathar's Guide to Everything store page and scroll down to to the Subclasses section.

Also note that a Subscription does not unlock content, for more info check out this thread: https://www.dndbeyond.com/forums/d-d-beyond-general/d-d-beyond-feedback/96069-hardcover-books-d-d-beyond-and-you-faq

Well i wondered if that was the case, thanks for the help. What doesnt help is having access to the actual books and not being able to use them on here.

You can use the homebrew tools if you have access to the physical books.

Sorry you gotta splash some cash for it. I wouldn’t bother because in my opinion it is overpriced.

If you are in a group with someone who has the Master tier subscription you and has that book, you could join their campaign and with content sharing turned on the character in the campaign will have access to it that way. 

Homebrew, as was mentioned, is another way to access it, well, a homebrew copy of it.