Plugin management page (wp-admin/plugins.php) NOT working

Hello, I get this message everytime I go to the plugin page. I manage several WordPress Sites and it only happens on one:

Plugins Fatal error: Call to undefined function self_admin_url() in /home/web/users/4039.talyst/sites/talystinsite/public_html/wp-admin/plugins.php on line 393

I viewed the file’s Line 393 and it seems normal:

<a href="<?php echo self_admin_url( 'plugin-install.php' ); ?>" class="button add-new-h2"><?php echo esc_html_x('Add New', 'plugin'); ?></a>

Would there be a conflicting plugin causing this?