How to fix the “Missing temporary folder” error?
To fix this error, you have to edit the wp-config.php file in WordPress. You can edit this file using file manager or FTP. The file will be there in the root directory of WordPress installation.
Find the wp-config.php file and edit it.

Copy paste the following code to the file before the line that says `That’s all, stop editing! Happy blogging’.

define('WP_TEMP_DIR', dirname(FILE) . '/wp-content/temp/');

Save your changes.
Go to /wp-content/ directory.
Create a new directory named temp.
Set dir permission to 755.
Check the WordPress site now.

If you are running Cloudlinux with Cagefs, then disable and enable Cagefs for the particular user, this will resolve the error:

cagefsctl --disable $USER
cagefsctl --enable $USER