"you cannot accept a challenge at this moment"

Why not?

I went to custom game and selected to play premium members.  One appears and when I click on the "play" button, I get a footer which reads "you cannot accept a challenge at this moment."

Why?

His rating seek probably doesn't correspond to your rating. That is the most predominant reason.
For instance: Let's say I click on some 1 500 rated person's challenge. If his rating seek is +50 points at maximum, I will not be able to play a game against him because he doesn't want it.
Or if I click on someone's challenge who is 2 200 rated, perhaps he has -200 rating range included in his preferences so he will not play rated games against anyone below 2 000 at that moment.

Might be your opponent's settings????

Understood. I will keep a closer eye on ratings, maybe that's it, but I could have sworn the last two players were +/- 50 points of my (rapidly declining) rating.

Thanks for the replies, friends. 👍

No problem. I think that there are even +/- 25 settings as well, at least there were in the past.

No. When it says you cannot accept the challenge, it means someone has accepted the challenge before you or the user has cancelled its challenge.
If you do not match the user match settings (-+-25, +-50 etc) the user will not show up in the open challenges. It is probably the opponent cancelled when you clicked it or someone accepted since there is probably some lag.

Good to know. It does seem to be moving rather quickly. Thank you, friend. 🙏

In my experience I am unsure if this is true. The reason why I say this is that I play in a pretty small pool (60 minutes per side, which is exceptionally small in 1 500+ rating range), and I had some opponents slightly weaker that I couldn't challenge on multiple different occasions, while seeing their challenges. 
Plus, in many occasions, the challenge stays for a few minutes, so I try to click on it many times during that time without success, and in my opinion it is unlikely for a challenge to stay visible for that long time if the user has canceled a challenge or someone else has accepted it.

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