You can add JavaScript code to the registration page to 

 1. hide the "Username" input element and 
 2. assign the entered "email address" to "username"

You need to create (if you don't already have) an override to the registration script. For step-by-step instructions on creating an override to scripts, you can refer to the following blog: https://www.joomlashack.com/blog/joomla-how-to/how-to-simplify-default-registration-form-in-joomla-3/ 

On Joomla 4, the registration script that needs to be updated follows.
 
`templates\<Your_Template_Name>\html\com_users\registration\default.php`

Apply the following changes to it. 

```javascript
<?php
defined('_JEXEC') or die;
// more statements...
?>

<!-- Add the following JavaScript code -->
<script type="text/javascript">
document.addEventListener('DOMContentLoaded', function () {
   // Find the email and username input fields
	var emailField = document.getElementById('jform_email1');
	var usernameField = document.getElementById('jform_username');
	var usernameLabel =	document.getElementById('jform_username-lbl');

	// Uncomment the following to make sure the correct elements are selected. If any of the following is null, check the IDs used above.
   // alert("all of the following should have values (NOT null)"
	// 	+ "\n - usernameField: " + usernameField
	// 	+ "\n - emailField: " + emailField
	// 	+ "\n - usernameLabel: " + usernameLabel);

	// Hide the username "input elemnet" and its "label"
	usernameField.parentElement.style.display = 'none';
	usernameLabel.parentElement.style.display = 'none';

   // Make sure the emailField exists
	if (emailField) {
		// Assign the value of the email field to the username field
		emailField.addEventListener('blur', function () {
			// Trim spaces
			var emailValue = emailField.value.trim();

			// Assign the email as the username
			usernameField.value = emailValue;
		});
	}
});
</script>

<div class="com-users-registration registration">
<!-- more statements --> 
</div>
```