This is uploading to a Genuino Uno, using Arduino Creator on ChromeOS, which I bought earlier today.
This is the log from the upload:
./opt/arduino-builder/arduino-builder -compile -core-api-version 10611 -build-path /tmp/479536879/build -hardware opt/arduino-builder/hardware -hardware ./opt/cores -tools opt/arduino-builder/tools -tools ./opt/tools -built-in-libraries opt/libraries/latest -libraries /tmp/479536879/pinned -libraries /tmp/479536879/custom -fqbn arduino:avr:uno -build-cache /tmp -logger humantags -verbose=false /tmp/479536879/sketch_mar10a
Sketch uses 444 bytes (1%) of program storage space. Maximum is 32256 bytes.
Global variables use 9 bytes (0%) of dynamic memory, leaving 2039 bytes for local variables. Maximum is 2048 bytes.
Validating license ...
What next steps should I take?
The 'Busy' icon is cycling. And the only thing I can do is cancel it.
Okay, when I reopened my laptop an hour later: I got a message asking me to provide permission to see my purchases in the store. I clicked through and now everything works.
So it seems the UI doesn't do a good job of giving feedback that it doesn't have permission to access the license information yet.