Player not found - Unable to Guild Invite or send Work Orders

I bought DF edition WoW for my friend. We joined a server togerther -we on the same server and both play Horde.
We have existing WoW accounts since before DF came out.
I joined a guild and cannot send him a Guild invite - just says player not found.
I tried to send him a Personal Work order to him/his account/characters. It doesnt come up in Auto Complete and also says player not found.
Can you please fix his account so that we can invite to the guild & send personal work orders?

His user ID is : Po0z#11740

FYI other guild members have tried to invite him and also get same error : Player not found. So unlikely to be an issue with my account.