I received an apology email from Audible about the UK codes I shared for a free copy of my audiobook, Friends and Enemies. This should explain why there were so many difficulties. I will reapply for new codes and post them here. Please accept my sincerest apologies along with that of Audible/ACX. Here is what their email said:

Dear Joy D King,
We are contacting you today regarding Promo Codes you may have generated for use on Audible.co.uk. We recently found an issue that affected the redemption of these
codes in the Audible UK store. We have fixed this issue, but unfortunately, we have
been forced to invalidate any codes issued for the UK store before 5/14/2019. We
sincerely apologize for the inconvenience this may cause you.

The ACX Team