"invalid user" error when trying to add a member to a private channel

Dear Team Community Members,

I am using Teams for my personal project and I'm having some issues with adding a member to a private channel. There is no problem adding him to a standard channel, but when I try adding him to a private one, it says invalid user- even though in standard channels the user is valid. Please see the screenshots attached. When I add him, it looks like he's been added, but he does not show up in the private channel members group nor does he get access to the channel. 

Any tips/guidance would be extremely helpful. Please note, this is for a free version of teams for my personal project. There is no problem adding other members to a private channel, it's just his account.