I can't post eye-catch images on WordPress post screen.

Prerequisites/What you want to achieve

I also want to post eye-catch images on the WordPress post screen.

Problems/Error Messages you are experiencing

"From the WordPress dashboard, the ""Add New Post"" screen appears, but the ""Eye Catch Image"" item that should appear in the lower right corner does not appear, so I cannot enter the eye catch image."

Source Codes Affected

functions.php contains the following:

<?php
/**
 * Enable eye-catch images
 */
add_theme_support('post_thumbnails');

Tried

From the WordPress dashboard, I opened View Options on the Add New Post screen under Post New, but the Eye Catch Image box is not displayed.

Supplementary information (e.g. language/FW/tool version)

Environment

• Latest version as of WordPress 4.8 2017.7.6
• Rental Server: Domain King P Plan
• PHP 7.0.10 (Run Type: FastCGI Application)
• MySQL 5.6 (database server 127.0.0.1:3316)
• Windows 10 HOME 64-bit 10.0.10011.16384
• Microsoft Edge 38.14393.0.0
• EdgeHTML 14.14393
• Googlechrome 57.0.2987.133