Usually, this means there are file permission issues. Please follow these steps:

A. Log into your web hosting account. Go to cPanel > File Manager, and open public_html.

B. We recommend browsing through the public_html folder and viewing file permissions collectively first.

The last column in cPanel shows a 3-digit number which indicates the file permissions:

If you see any file or folder that has a number that’s different from this recommendation, you can fix it easily. Select the file or folder, and then click on Permissions.

Enter the correct permission settings. Make sure you stick to the recommended file permissions which are 644 or 640 for WordPress files, and 755 or 750 for directories or folders.

Click on Change Permissions. Now visit the page in question and see if you have regained access.

If the issue persists please:

Upgrade PHP to the latest version
Revert your site to the pre-error state, if you have a backup file
Use WP_Debug to get a list of errors and see if anything stands out
Review your server’s access and error log and see if anything stands out
Deactivate themes and plugins.
Manually reset your .htaccess file.
Double-check your site is using the correct database prefix