Use `chsh` on GCP VM in order to change from bash to zshell, default password is unknown

I'm trying to change the default shell on a GCP VM but am running into difficulties trying to use chsh. It seems that I have access to root as I am able to run sudo apt upgrade.

I have also installed zshell:

user_name@instance_name:~$ which zsh
/usr/bin/zsh

But when trying to change the shell I get the following:

user_name@instance_name:~$ chsh -s $(which zsh)
Password:

And I have no idea what the password is, I tried just pressing RET but got:

user_name@instance_name:~$ chsh -s $(which zsh)
Password:
chsh: PAM: Authentication failure

And am not sure what I'm supposed to do. The output of $USER (if it's of use) is the following:

user_name@instance_name:~$ echo $USER
user_name

The instance was created with the following command:

gcloud compute instances create $INSTANCE_NAME \
        --zone=$ZONE \
        --image-family=$IMAGE_FAMILY \
        --image-project=deeplearning-platform-release \
        --maintenance-policy=TERMINATE \
        --accelerator="type=nvidia-tesla-p100,count=1" \
        --machine-type=$INSTANCE_TYPE \
        --boot-disk-size=200GB \
        --metadata="install-nvidia-driver=True" \
        --preemptible

If there's any other information that I should share please let me know.

So GCP VM has no default password and you should set it first. For this you can do something like this:

sudo passwd <user>

After setting the password, you can now set the default shell to zsh by:

chsh -s $(which zsh) <user>

Enter the new password and the default shell should be changed.