Hi.
I have the same problem.
I tried some solution to solve this problem on the chromebook.
The arduino IDE using linux(beta) on the chromebook does not recognized the arduino uno.
First, a powerwash of ChromeOS. -> nothing has worked. A powerwash didn't completely downgrade it.
Second, changed channel to beta version. -> nothing has worked. The arduino web editor does not boot up well in the beta version.
Finaly, changed channels back to staible. -> A powerwash was automatically performed, and ChromeOS version was downgrad to v72. Then, the arduino create recognized the arduino uno.
However, ChromeOS v72 began v85 by automatically OS update. Then, the arduino create does not recognized the arduino uno.
I found dev/ttyACM0 not working properly. So that, tha arduino IDE and ithe arduino create does not recognized the arduino machine in ChromeOS v85.
I think the only solution is to change ChromeOS to v72 or wait for the ChromeOS more update.
It seems to be possible to fix the version of ChromeOS for a fee (google suite).