Under the drop down the only option for a patron is 'the fiend'

Also I can't take the 'book of ancient secrets' invocation even though I have selected the pact of the tome feature.

For the beta test, only material from the SRD and Basic Rules is available. The other official material will be added at the final release. The price for that is still to be announced.

http://www.dndbeyond.com/forums/d-d-beyond-general/beta-testing-feedback/568-beta-phase-one-basic-rules-content-only#c1

what's weird is it shows the book of ancient secrets on the list of options but doesn't appear as a drop down option.

Conversely, the list only mentions 'the archfey' as a patron, but only 'the fiend' is an option.

For the archfey thing, it is bit of weird wording from the SRD. But, it is only the fiend that we get for the beta test.

For the book of ancient thing, I would advice to submit it here:

http://www.dndbeyond.com/forums/d-d-beyond-general/beta-testing-feedback/3649-mega-thread-focused-feedback-character-builder?page=17#c349