This issue is most commonly caused by wrong file permissions. Your theme and upload directory should have correct file permissions otherwise WordPress will not be able to store your files in it or sometimes may not even display files from the directory. You would need a FTP client, like Filezilla.
Important Notice: After you update the file permissions and test the SUBSCRIBE button you just need to change file permissions back to normal (755).
Connect to your website using the FTP client and find /wp-content/themes/theme_root/theme_config directory. Right click on the theme_config directory and select file permissions. Watch a video how to do this
A new dialog box for file permissions will open. You will need to set file permissions for wp-content directory. To do that, enter 777 in the numeric value box. Click on the OK button to apply these changes. Filezilla will now start applying file permissions to the directory.
Now test the SUBSCRIBE button. After that don’t forget to set the permissions back to 755 to avoid any security issues. Watch a video how to do this Also be sure not to modify or delete any of the content in your home directory.