I'm trying to create an Elven Archer using a Wood Elf and the Battle Master Build - Archer but cannot seem to see either the Elven Accuracy feat from XGtE or the Piercer feat from TCoE when using the Character Builder. I've tried using every combination of the Optional Features and Use Prerequisites but no luck so far.

Please help. 

Do you own those sourcebooks on DDB? Or if not, do you own at least those feats from those books? Or if not, are they being shared with you? Those feats are not free content.

To have access to those feats, you need to have purchased those feats on Beyond, or purchased those books, or be in a campaign with someone sharing their content with you.

Make sure your entitlements are in sync. You can navigate to that by clicking on the link in my signature.

Try turning the prequisite option OFF. The slider should appear gray and the circle is towards the left.

I own both sourcebooks on DnDB and gave tried prerequisites off.

Double check that you bought the full book and not the compendium version (which is cheaper but essentially read-only and doesn't unlock anything in the character builder).

Yup, full book purchased.

Is your character at least 4th level? Feats are typically only gained in lieu of an ASI, so unless you have achieved a level that grants an ASI, you won’t be able to add a feat through the builder.

10th level.

Hover your mouse over the "Astroverdia" in the top right corner of your screen and click on 'Account'.

The check the Marketlace tab and make sure it says, 

Account Entitlements

Your entitlements data is in sync! You should have nothing to worry about, but contact our Support Team if you're having problems accessing your purchased content.

If it has an option to sync instead, use that option.

Aso, scroll down and make sure the books you purchased are showing below on that screen and reach out to support if they're not.

Ah, hell! Found it. Didn't think to look for Feat under the Ability Score Improvement.