Pixel XL with Android 10 (build QP1A.191005.007.A1). Enabled Backup, selected a backup account (only one account on the device), and found the ‘Back up now’ button was grayed out. There is a strange work around — temporarily removing the security pin. The ‘Back up now’ button is active, and a backup can be performed successfully.
*But* I have to re-register my fingerprints when the pin is disabled / re-enabled. Which means I’ve got to type my long and complex banking password on the little phone keyboard. Not cool. I was able to back up my phone using ADB but needed to input a passphrase on the device to encrypt the backup. Not sure if there’s something about the screen lock security that requires the backup to be encrypted … which then precludes the magic to-the-cloud backup from proceeding?
To back up device from command line:
adb backup -apk -shared -all -f ./backup.ab
< on device, type a passphrase and approve the device backup>