Thanks! the device is a google pixel 3a, and my computer is a mac.
It remained frozen all night, so I tried closing the installer, then reopening it. When I did that, it initially no longer could detect my phone (though I had checked last night that the phone was detected).
The phone had been in fastboot mode, so I tried clicking "start" on it, and it loaded android 9. Then it was detected (it showed up when I typed "adb devices" in terminal).
So now I attempted it again, this time selecting "stable" instead of "devel" when selecting options. Both times I selected "wipe device."
When I attempted install again, it immediately said "the installation stopped due to a problem. I'll try to attach a screenshot but the error message is "Error: unknown: TypeError: Cannot read properties of undefi.."
The phone also switched back into the screen that says "fastboot mode" and it verifies that it is unlocked. It's a red triangle with an error. The last two lines say:
Device state: unlocked
Enter reason: reboot bootloader