Am completely unable to find the manage feats button. Seems to not exist for me. Help definitely appreciated, 

Thanks 

You have to go to the “Features & Traits” tab of the character sheet, then click/tap “Feats” and you will find “Manage Feats” there.

Well there goes like a wasted 2 hours. I thought that was a heading, didn't realize it was clickable. Thank you!

I found the manage feat button but it won't open the the whole feat guide from the player's handbook. Did I do something wrong when making the character sheet?

Not all feats from the PHB are free content, only the one feat is. The rest must be purchased either by purchasing the whole PHB, or by purchasing the feats pack from that book: (https://www.dndbeyond.com/marketplace/sourcebooks/players-handbook).

I have a question along the same lines.

My DM gave us the right to choose a free feat for a level 1 character. For my barbarian I choose the crusher feat which allows to add a +1 to either strength or Constitution. Now on my character sheet it says that I have this free +1 and that I have to choose on which stat to apply it.

I found a way to add a +1 to the stat I wanted by going back to the character builder and doing it manually, but on my character sheet it still says "No choice made". Is there a way to solve this ?

On the character sheet, go to the features & traits tab and click on 'manage feats.' Expand the Crusher feat (which should have an exclamation mark in a blue circle next to it, indicating you need to make a selection) by clicking on the down arrow. There should be a dropdown menu to make your selection.

Thanks ! It's so simple, I don't know how I could miss it !