Make sure the following requirements are met to upload excel file.
- PHP version 5.6 or newer
- PHP extension php_zip enabled
- PHP extension php_xml enabled
- PHP extension php_mbstring enabled
- PHP extension php_gd2 enabled (if not compiled in)
If you are uploading big excel file with thousands of messages then make sure your server has big enough max_execution_time and memory_limit values.
If it still doesn't work after following the above instructions. Look for errors related to the SMS Gateway app in your PHP error_log file and submit a ticket with the error message. If your error_log file doesn't have any errors related to SMS Gateway then it must be an issue with your server and in that case, you should contact your server administrator about the issue.