OK - here's how development REALLY works (and you all know it's true)...
I got STUCk with this same @#$%^& problem...
I beat my head against the wall trying to understand the root cause...
i read FORUM postings...
I perused PowerApps Community discussions...
I even read the PowerApps DOCUMENTATION...
... but no luck.
I read the beginning of this post which (annoyingly) reminded me to select the datacard-as-container for the button I wished to add: thanks dad - DID that.
STILL NO LUCK - the **bleep** button still DIDN'T show up as part of the CARD 😠 !!!
THEN, after I exhausted ALL my technical experience and technical know-how and technical troubleshooting...
..my wife (of 26+ years) came downstairs, and announced she'd just been on a single phone call so long she killed her phone...
AND SUDDENLY I REALIZED I HAD NOT YET "UNLOCKED" THE DATACARD CONTROL !!!
I selected the control, UNLOCKED it, and then copied/pasted the offending button into the container..
... IT PASTED IN PERFECTLY.
Yup - wife-killing-phone-with-epic-long-conversation-stimulates-epiphany-that-container-control-needs-UNlocking.
Intuitive, don't you think? 😉 But this is how development REALLY works sometimes (all the time) 😉 ...
Hope this helps everyone else who's NOT-QUITE-GOT-THIS yet.
UNLOCK YOUR CONTROL BEFORE INSERTING OR PASTING.
Cheers,
-Mark Vogt, PowerApps Developer Green Belt (heading toward Blue) 😉