I'm trying to edit the sheet of one of my players in the campaign I'm running but anytime I try to adjust something (in this case adding feat and spell to that character) I keep getting this error message. Any insight? 

Yes, there’s a bug that lets DMs see the stuff they own when looking at a player’s sheet, even if the player doesn’t have access to that content. Do you or any of the players have a Master Tier account and have activated content sharing? If not then one of you needs to or you won’t be able to use your content on their sheets.