SSH connection works only with IP address and not domain name

I created a new Linode and set up SSH, but can only log in with the IP address and not the domain name.

This command logs me in: SSH example_user@<IP4>
This command generates an error: SSH example_user@domain.com

Error message:
ssh: Could not resolve hostname domain.com: This is usually a temporary error during hostname resolution and means that the local server did not receive a response from an authoritative server.

STEPS TO SET UP THE SERVER:

1. Server setup:
   Distribution: Debian 12
   Region: Chicago, IL (us-ord)
   Linode Plan: Shared CPU > Nanode 1GB $5
   Linode Label: debian-us-ord-domain
   Add Tags: <blank>
   Root Password: <entered password>
   SSH Keys: <laptop label>
   Assign VPC: None
   Assign Firewall: None
   <Left everything else blank for the setup>

2. Domain setup for Hostname, IP Address, and TTL:
   domain.com <IP4 address> Default domain.com <IP6 address> Default mail <IP4 address> Default mail <IP6 address> Default www <IP6 address> Default www <IP4 address> Default

3. Connect to the server through SSH:
   ssh root@<IP4>

4. Windows 11 known_hosts file has legitimate IP addresses and no obsolete IP addresses

5. Update and upgrade the packages
   sudo apt update && sudo apt upgrade

6. Restart SSH on Debian 12 server
   sudo systemctl restart ssh

7. Set the timezone
   timedatectl set-timezone 'America/Chicago'

8. Open the hosts file with Nano.
   nano /etc/hosts

9. Add the <ip4> and <ip6> lines to the /etc/hosts file after the 127.0.0.1 where <ip4> is the xxx.xxx.xxx.xxx numeric address and <ip6> is the xxxx.xxxx.xxxx.xxxx.xxxx.xxxx alphanumeric address:
   127.0.0.1 localhost.localdomain localhost <ip4> www.boldpostprojects.com boldpostprojects.com <ip6> www.boldpostprojects.com boldpostprojects.com

10. Create the user name (I replaced example_user with a user name):
    adduser example_user

11. Add example_user to the sudo group so it will have root user capabilities:
    adduser example_user sudo

12. Log back into the server
    ssh example_user@<IP4>

13. Create the authorized_keys file to store the SSH public key on the server side:
    sudo mkdir -p ~/.ssh && cd .ssh && sudo touch authorized_keys && cd .. && sudo chmod -R 700 ~/.ssh/ && sudo chmod 600 ~/.ssh/authorized_keys

14. Exit from the server
    exit

15. From the client side (the laptop), copy the public SSH key from the laptop to the server:
    scp C:\Users\sbrew\.ssh\id_ed25519.pub example_user@172.234.18.254:~/.ssh/authorized_keys

16. Reboot the server

17. Log back into the server
    ssh example_user@domain.com

I get the following error:
ssh: Could not resolve hostname domain.com: This is usually a temporary error during hostname resolution and means that the local server did not receive a response from an authoritative server.

I am able to log in successfully with ssh example_user@<ip4>
It seems that the addition of the following lines in the /etc/hosts file are what is needed to log in with the following line:
ssh example_user@domain.com

What am I missing?