The purchase of a physical book does not get you rights on the access of the digital version of the book.

If you want to use character options here in DDB, you have two possibilities:

1) You need to buy the sourcebook, or the single option (class, race, spell, item...).

2) You can homebrew the option yourself with no cost.

To clarify, you can homebrew subclasses, not classes from the ground up.

https://www.dndbeyond.com/homebrew/creations/create-subclass

You can do it on the link above, and I advise you to start from an existing subclass, modifying the features.