Sometimes these types of issues are transient and the following SSH connection attempt succeeds if the instance was still booting up during the initial attempt.
The error you found in the auth.log is very generic, you can review the serial console output to view some additional details to help you troubleshoot the problem.
If you identify the possible root cause of the issue, you can enable interactive access to the serial console to fix it.
If you don’t have the root password, you can:
1. Go to the VM instances page in Google Cloud Platform console.
2. Click on the instance for which you want to add a startup script.
3. Click the Edit button at the top of the page.
4. Click on ‘Enable connecting to serial ports’
5. Under Custom metadata, click Add item.
6. Set 'Key' to 'startup-script' and set 'Value' to this script:
useradd -G sudo USERNAME
echo 'USERNAME:PASSWORD' | chpasswd
7. Click Save and then click RESET on the top of the page. You might need to wait for some time for the instance to reboot.
8. Click on 'Connect to serial port' in the page.
9. In the new window, you might need to wait a bit and press on Enter of your keyboard once; then, you should see the login prompt.
10.. Login using the USERNAME and PASSWORD you provided.
Serial console access is not secure so it is recommended that you disable it once you complete your work.
You can also debug this by running SSH Diagnosis scripts from a Linux Guest environment which are included with Linux instances created using Google-provided public images.