This question was answered by @jesin:

Hi @renatov, as you created a Droplet using the SSH key you’ll have to use the key to login. On that article you’ve linked, read below the Setting Up an SSH Session with SSH Keys in PuTTY section. Once logged in you can use the passwd command to set a root password.

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Hello there,

If you’re unable to access your droplet you can try to reset the root password and then check your email and access the droplet via the web console.

You can reset your root password for your account. Navigate to the control panel. From the project, the Droplet is in, or from the main navigation’s Droplets page, locate the Droplet. Click the Droplet’s name to open its detail page, then select Access in the left navigation.

Click Reset Root Password to reset the Droplet’s password. You will receive an email containing the Droplet’s temporary password.

Note: Some operating systems use internally-managed passwords, which means you cannot reset the root password from the control panel. In these cases, you’ll see the following message in the Reset root password section:

Once you have received your new password you can either ssh to the droplet using your ssh client or access the droplet using the DigitalOcean console:

https://www.digitalocean.com/docs/droplets/resources/console/

Log In with the Console

From the DigitalOcean Control Panel, first click the name of the Droplet you want to access, then select Access from the left navigation. Click the Launch Console button to open a web-based console session.

When the console opens, click the console screen and press ENTER to ensure that the login prompt has focused. At the login prompt, enter the user you want to log in as. This is typically the root user or a user-configured with sudo privileges.

When prompted, enter the password associated with the account. For security purposes, your password will not be displayed on the screen as you type or paste it into the command line.

Once you’ve entered the password, press ENTER. If this is the first time you’ve logged in after resetting the root password through the control panel, you are prompted to enter the password again, then asked to choose a new password.

Once you’ve successfully entered your credentials you should be logged in to your Droplet.

Hope that helps!
Regards,
Alex